FLUID POWER

Figure 2-1.—Device for determining the arrangement of the force, pressure, and area formula. Figure 2-1 illustrates a memory device for recalling the different variations of this formula. Any letter in the triangle may be expressed as the product or quotient of the other two, depending on its position within the triangle. For example, to find area, consider the letter A as being set off to itself, followed by an equal sign. Now look at the other two letters. The letter F is above the letter P; therefore, NOTE: Sometimes the area may not be expressed in square units. If the surface is rectangular, you can determine its area by multiplying its length (say, in inches) by its width (also in inches). The majority of areas you will consider in these calculations are circular in shape. Either the radius or the diameter may be given, but you must know the radius in inches to find the area. The radius is one-half the diameter. To determine the area, use the formula for finding the area of a circle. This is written A = where A is the area, 3.1416 (3.14 or 3 1/7 for most calculations), and r 2 indicates the radius squared. Atmospheric Pressure The atmosphere is the entire mass of air that surrounds the earth. While it extends upward for about 500 miles, the section of primary interest is the portion that rests on the earth’s surface and extends upward for about 7 1/2 miles. This layer is called the troposphere. If a column of air 1-inch square extending all the way to the “top” of the atmosphere could be weighed, this column of air would weigh approximately 14.7 pounds at sea level. Thus, atmospheric pressure at sea level is approximately 14.7 psi. As one ascends, the atmospheric pressure decreases by approximately 1.0 psi for every 2,343 feet. However, below sea level, in excavations and depressions, atmospheric pressure increases. Pressures under water differ from those under air only because the weight of the water must be added to the pressure of the air. Atmospheric pressure can be measured by any of several methods. The common laboratory method uses the mercury column barometer. The height of the mercury column serves as an indicator of atmospheric pressure. At sea level and at a temperature of 0° Celsius (C), the height of the mercury column is approximately 30 inches, or 76 centimeters. This represents a pressure of approximately 14.7 psi. The 30-inch column is used as a reference standard. Another device used to measure atmospheric pressure is the aneroid barometer. The aneroid barometer uses the change in shape of an evacuated metal cell to measure variations in atmospheric pressure (fig. 2-2). The thin metal of the aneroid cell moves in or out with the variation of pressure on its external surface. This movement is transmitted through a system of levers to a pointer, which indicates the pressure. The atmospheric pressure does not vary uniformly with altitude. It changes more rapidly at lower altitudes because of the compressibility of the air, which causes the air layers close to the earth’s surface to be compressed by the air masses above them. This effect, however, is partially counteracted by the contraction of the upper Figure 2-2.—Simple diagram of the aneroid barometer. 2-2 F A P FPf02001 COUNTERWEIGHTPOINTER SECTOR GEAR ANEROID CELL ADJUSTING SPRING FPf02002

layers due to cooling. The cooling tends to increase the density of the air. Atmospheric pressures are quite large, but in most instances practically the same pressure is present on all sides of objects so that no single surface is subjected to a great load. Atmospheric pressure acting on the surface of a liquid (fig. 2-3, view A) is transmitted equally throughout the liquid to the walls of the container, but is balanced by the same atmospheric pressure acting on the outer walls of the container. In view B of figure 2-3, atmospheric pressure acting on the surface of one piston is balanced by the same pressure acting on the surface of the other piston. The different areas of the two surfaces make no difference, since for a unit of area, pressures are balanced. TRANSMISSION OF FORCES THROUGH LIQUIDS When the end of a solid bar is struck, the main force of the blow is carried straight through the bar to the other end (fig. 2-4, view A). This happens because the bar is rigid. The direction of the blow almost entirely determines the direction of the transmitted force. The more rigid Figure 2-4.—Transmission of force: (A) solid; (B) fluid. the bar, the less force is lost inside the bar or transmitted outward at right angles to the direction of the blow. When a force is applied to the end of a column of confined liquid (fig. 2-4, view B), it is transmitted straight through to the other end and also equally and undiminished in every direction throughout the column—forward, backward, and sideways—so that the containing vessel is literally filled with pressure. An example of this distribution of force is illustrated in figure 2-5. The flat hose takes on Figure 2-3.—Effects of atmospheric pressure. Figure 2-5.—Distribution of force. 2-3 14.7 LBS. PER SQ. INCH 14.7 14.7 14.7 14.7 14.7 14.7 14.7 LBS. PER SQ. INCH 14.7 A B FPf02003 B FPf02004 A FPf02005

a circular cross section when it is filled with water under pressure. The outward push of the water is equal in every direction. So far we have explained the effects of atmospheric pressure on liquids and how external forces are distributed through liquids. Let us now focus our attention on forces generated by the weight of liquids themselves. To do this, we must first discuss density, specific gravity, and Pascal’s law. Density and Specific Gravity The density of a substance is its weight per unit volume. The unit volume in the English system of measurement is 1 cubic foot. In the metric system it is the cubic centimeter; therefore, density is expressed in pounds per cubic foot or in grams per cubic centimeter. To find the density of a substance, you must know its weight and volume. You then divide its weight by its volume to find the weight per unit volume. In equation form, this is written as Equation 2-4. EXAMPLE: The liquid that fills a certain container weighs 1,497.6 pounds. The container is 4 feet long, 3 feet wide, and 2 feet deep. Its volume is 24 cubic feet (4 ft x 3 ft x 2 ft). If 24 cubic feet of this liquid weighs 1,497.6 pounds, then 1 cubic foot weighs or 62.4 pounds. Therefore, the density of the liquid is 62.4 pounds per cubic foot. This is the density of water at 4°C and is usually used as the standard for comparing densities of other substances. The temperature of 4°C was selected because water has its maximum density at this temperature. In the metric system, the density of water is 1 gram per cubic centimeter. The standard temperature of 4°C is used whenever the density of liquids and solids is measured. Changes in temperature will not change the weight of a substance but will change the volume of the substance by expansion or contraction, thus changing the weight per unit volume. In physics, the word specific implies a ratio. Weight is the measure of the earth’s attraction for a body. The earth’s attraction for a body is called gravity. Thus, the ratio of the weight of a unit volume of some substance to the weight of an equal volume of a standard substance, measured under standard pressure and temperature con- ditions, is called specific gravity. The terms specific weight and specific density are sometimes used to express this ratio. The following formulas are used to find the specific gravity (sp gr) of solids and liquids, with water used as the standard substance. or, The same formulas are used to find the specific gravity of gases by substituting air, oxygen, or hydrogen for water. If a cubic foot of a certain liquid weighs 68.64 pounds, then its specific gravity is 1.1, Thus, the specific gravity of the liquid is the ratio of its density to the density of water. If the specific gravity of a liquid or solid is known, the density of the liquid or solid maybe obtained by multiplying its specific gravity by the density of water. For example, if a certain hydraulic liquid has a specific gravity of 0.8, 1 cubic foot of the liquid weighs 0.8 times as much as a cubic foot of water—0.8 times 62.4, or 49.92 pounds. In the metric system, 1 cubic centimeter of a substance with a specific gravity of 0.8 weighs 1 times 0.8, or 0.8 grams. (Note that in the metric system the specific gravity of a liquid or solid has the same numerical value as its density, because water weighs 1 gram per cubic centimeter.) Specific gravity and density are independent of the size of the sample under consideration and depend only on the substance of which it is made. A device called a hydrometer is used for measuring the specific gravity of liquids. 2-4

FPf02007 WATER WEIGHT 62.4 lb. WATER PRESSURE 62.4 LB./ft 2 0.433 lb./in 2 FPf02006 Pascal’s Law Recall from chapter 1 that the foundation of modern hydraulics was established when Pascal discovered that pressure in a fluid acts equally in all directions. This pressure acts at right angles to the containing surfaces. If some type of pressure gauge, with an exposed face, is placed beneath the surface of a liquid (fig. 2-6) at a specific depth and pointed in different directions, the pressure will read the same. Thus, we can say that pressure in a liquid is independent of direction. Pressure due to the weight of a liquid, at any level, depends on the depth of the fluid from the surface. If the exposed face of the pressure gauges, figure 2-6, are moved closer to the surface of the liquid, the indicated pressure will be less. When the depth is doubled, the indicated pressure is doubled. Thus the pressure in a liquid is directly proportional to the depth. Consider a container with vertical sides (fig. 2-7) that is 1 foot long and 1 foot wide. Let it be filled with water 1 foot deep, providing 1 cubic foot of water. We learned earlier in this chapter that 1 cubic foot of water weighs 62.4 pounds. Using this information and equation 2-2, P = F/A, we can calculate the pressure on the bottom of the container. Since there are 144 square inches in 1 square foot, This can be stated as follows: the weight of a column of water 1 foot high, having a cross- sectional area of 1 square inch, is 0.433 pound. If the depth of the column is tripled, the weight of the column will be 3 x 0.433, or 1.299 pounds, and the pressure at the bottom will be 1.299 lb/in 2 (psi), since pressure equals the force divided by the area. Thus, the pressure at any depth in a liquid is equal to the weight of the column of liquid at that depth divided by the Figure 2-6.—Pressure of a liquid is independent of direction. cross-sectional area of the column at that depth. The volume of a liquid that produces the pressure is referred to as the fluid head of the liquid. The pressure of a liquid due to its fluid head is also dependent on the density of the liquid. If we let A equal any cross-sectional area of a liquid column and h equal the depth of the column, the volume becomes Ah. Using equation 2-4, D = W/V, the weight of the liquid above area A is equal to AhD. Figure 2-7.—Water pressure in a 1-cubic-foot container. 2-5

FORC E 1 = 100 LBS. FORC E 2 = 100 LBS. INPUT PISTON 10 SQ. IN. OUTPUT PISTON 10 SQ. IN. PRESSURE 10 LBS PER SQ. INCH FPf02009 A H SURFACE HEAD FPf02008 Since pressure is equal to the force per unit area, set A equal to 1. Then the formula pressure becomes P=hD Equation 2-5. It is essential that h and D be expressed in similar units. That is, if D is expressed in pounds per cubic foot, the value of h must be expressed in feet. If the desired pressure is to be expressed in pounds per square inch, the pressure formula, equation 2-5, becomes Equation 2-6. Pascal was also the first to prove by experiment that the shape and volume of a container in no way alters pressure. Thus in figure 2-8, if the pressure due to the weight of the liquid at a point on horizontal line H is 8 psi, the pressure is 8 psi everywhere at level H in the system. Equation 2-5 also shows that the pressure is independent of the shape and volume of a container. Pressure and Force in Fluid Power Systems Figure 2-9.—Force transmitted through fluid. of the shape of the container. Consider the effect of this in the system shown in figure 2-9. If there is a resistance on the output piston and the input piston is pushed downward, a pressure is created through the fluid, which acts equally at right angles to surfaces in all parts of the container. If force 1 is 100 pounds and the area of the input piston is 10 square inches, then the pressure in the fluid is 10 psi Recall that, according to Pascal’s law, any force applied to a confined fluid is transmitted in all directions throughout the fluid regardless NOTE: Fluid pressure cannot be created without resistance to flow. In this case, resistance Figure 2-8.—Pressure relationship 2-6 with shape.

FORCE 1 = 20 LBS. FORCE 2 = 200 LBS. INPUT PISTON 2 SQ. IN. OUTPUT PISTON 20 SQ. IN. PRESSURE 10 LBS PER SQ. INCH 1" 1/10" FPf02011 FPf02010 FORCE 1 = 100 LBS. FORCE 2 = 100 LBS. INPUT PISTON 10 SQ. IN. OUTPUT PISTON 10 SQ. IN. PRESSURE 10 LBS PER SQ. INCH is provided by the equipment to which the output piston is attached. The force of re- sistance acts against the top of the output piston. The pressure created in the system by the input piston pushes on the underside of the output piston with a force of 10 pounds on each square inch. In this case, the fluid column has a uniform cross section, so the area of the output piston is the same as the area of the input piston, or 10 square inches. Therefore, the upward force on the output piston is 100 pounds (10 psi x 10 sq. in.), the same as the force applied to the input piston. All that was accomplished in this system was to transmit the 100-pound force around the bend. However, this principle under- lies practically all mechanical applications of fluid power. At this point you should note that since Pascal’s law is independent of the shape of the container, it is not necessary that the tube connecting the two pistons have the same cross-sectional area of the pistons. A connection of any size, shape, or length will do, as long as an unobstructed passage is provided. Therefore, the system shown in figure 2-10, with a relatively small, bent pipe connecting two cylinders, will act exactly the same as the system shown in figure 2-9. MULTIPLICATION OF FORCES.— Con- sider the situation in figure 2-11, where the input piston is much smaller than the output piston. Assume that the area of the input piston is 2 square inches. With a resistant force on the output piston a downward force of 20 pounds acting on the input piston creates a pressure of or 10 psi Figure 2-10.—Transmitting force through a small pipe. Figure 2-11.—Multiplication of forces. in the fluid. Although this force is much smaller than the force applied in figures 2-9 and 2-10, the pressure is the same. This is because the force is applied to a smaller area. This pressure of 10 psi acts on all parts of the fluid container, including the bottom of the output piston. The upward force on the output piston is 200 pounds (10 pounds of pressure on each square inch). In this case, the original force has been multiplied tenfold while using the same pressure in the fluid as before. In any system with these dimensions, the ratio of output force to input force is always ten to one, regardless of the applied force. For example, if the applied force of the input piston is 50 pounds, the pressure in the system will be 25 psi. This will support a resistant force of 500 pounds on the output piston. The system works the same in reverse. If we change the applied force and place a 200-pound force on the output piston (fig. 2-11), making it the input piston, the output force on the input piston will be one-tenth the input force, or 20 pounds. (Sometimes such results are desired.) Therefore, if two pistons are used in a fluid power system, the force acting on each piston is directly proportional to its area, and the magnitude of each force is the product of the pressure and the area of each piston. Note the white arrows at the bottom of figure 2-11 that indicate up and down movement. The movement they represent will be explained later in the discussion of volume and distance factors. 2-7

40 LBS.3 20 LBS. SQ. INCH AREA 6 SQ. IN. AREA 2 SQ. IN. 21 1. PISTON 2. CYLINDER 3. PISTON ROD FPf02012 DIFFERENTIAL AREAS.— Consider the special situation shown in figure 2-12. Here, a single piston (1) in a cylinder (2) has a piston rod (3) attached to one of its sides. The piston rod extends out of one end of the cylinder. Fluid under pressure is admitted equally to both ends of the cylinder. The opposed faces of the piston (1) behave like two pistons acting against each other. The area of one face is the full cross-sectional area of the cylinder, say 6 square inches, while the area of the other face is the area of the cylinder minus the area of the piston rod, which is 2 square inches. This leaves an effective area of 4 square inches on the right face of the piston. The pressure on both faces is the same, in this case, 20 psi. Applying the rule just stated, the force pushing the piston to the right is its area times the pressure, or 120 pounds (20 x 6). Likewise, the force pushing the piston to the left is its area times the pressure, or 80 pounds (20 x 4). Therefore, there is a net unbalanced force of 40 pounds acting to the right, and the piston will move in that direction. The net effect is the same as if the piston and the cylinder had the same cross-sectional area as the piston rod. VOLUME AND DISTANCE FACTORS.— You have learned that if a force is applied to a system and the cross-sectional areas of the input and output pistons are equal, as in figures 2-9 and 2-10, the force on the input piston will support an equal resistant force on the output piston. The pressure of the liquid at this point is equal to the force applied to the input piston divided by the piston’s area. Let us now look at what happens when a force greater than the resistance is applied to the input piston. In the system illustrated in figure 2-9, assume that the resistance force on the output piston is 100 psi. If a force slightly greater than 100 pounds is applied to the input piston, the pressure in the system will be slightly greater than 10 psi. This increase in pressure will overcome the resistance force on the output piston. Assume that the input piston is forced downward 1 inch. The movement displaces 10 cubic inches of fluid. The fluid must go somewhere. Since the system is closed and the fluid is practically incompressible, the fluid will move to the right side of the system. Because the output piston also has a cross-sectional area of 10 square inches, it will move 1 inch upward to accommodate the 10 cubic inches of fluid. You may generalize this by saying that if two pistons in a closed system have equal cross-sectional areas and one piston is pushed and moved, the other piston will move the same distance, though in the opposite direction. This is because a decrease in volume in one part of the system is balanced by one equal increase in volume in another part of the system. Apply this reasoning to the system in figure 2-11. If the input piston is pushed down a distance Figure 2-12.—Differential areas on a piston. 2-8

16 SQ. IN. 4 SQ. IN. VEL = 4 B A VEL = 1 FPf02013 of 1 inch, the volume of fluid in the left cylinder will decrease by 2 cubic inches. At the same time, the volume in the right cylinder will increase by 2 cubic inches. Since the diameter of the right cylinder cannot change, the piston must move upward to allow the volume to increase. The piston will move a distance equal to the volume increase divided by the surface area of the piston (equal to the surface area of the cylinder). In this example, the piston will move one-tenth of an inch (2 cu. in. ÷ 20 sq. in.). This leads to the second basic rule for a fluid power system that contains two pistons: The distances the pistons move are inversely proportional to the areas of the pistons. Or more simply, if one piston is smaller than the other, the smaller piston must move a greater distance than the larger piston any time the pistons move. LIQUIDS IN MOTION In the operation of fluid power systems, there must be a flow of fluid. The amount of flow will vary from system to system. To understand fluid power systems in action, it is necessary to understand some of the characteristics of liquids in motion. Liquids in motion have characteristics dif- ferent from liquids at rest. Frictional resistances within a fluid (viscosity) and inertia contribute to these differences. (Viscosity is discussed in chapter 3.) Inertia, which means the resistance a mass offers to being set in motion, will be discussed later in this section. There are other relationships of liquids in motion with which you must become familiar. Among these are volume and velocity of flow, flow rate and speed, laminar and turbulent flow, and more importantly, the force and energy changes which occur in flow. VOLUME AND VELOCITY OF FLOW The volume of a liquid passing a point in a given time is known as its volume of flow or flow rate. The volume of flow is usually expressed in gallons per minute (gpm) and is associated with relative pressures of the liquid, such as 5 gpm at 40 psi. The velocity of flow or velocity of the fluid is defined as the average speed at which the fluid moves past a given point. It is usually expressed in feet per second (fps) or feet per minute (fpm). Velocity of flow is an important consideration in sizing the hydraulic lines. (Hydraulic lines are discussed in chapter 5.) Volume and velocity of flow are often considered together. With other conditions unaltered—that is, with volume of input unchanged—the velocity of flow increases as the cross section or size of the pipe decreases, and the velocity of flow decreases as the cross section increases. For example, the velocity of flow is slow at wide parts of a stream and rapid at narrow parts, yet the volume of water passing each part of the stream is the same. In figure 2-13, if the cross-sectional area of the pipe is 16 square inches at point A and 4 square inches at point B, we can calculate the relative velocity of flow using the flow equation Q=vA Equation 2-7. where Q is the volume of flow, v is the velocity of flow and A is the cross-sectional area of the liquid. Since the volume of flow at point A, Q 1, is equal to the volume of flow at point B, Q 2, we can use equation 2-7 to determine the ratio of the Figure 2-13.—Volume and velocity of flow. 2-9

FPf02015 FPf02014 velocity of flow at point A, v 1, to the velocity of flow at point B, v 2. Since Q 1 = Q 2,A 1v 1 =A 2v 2 From figure 2-13; A 1 = 16sq. in., A 2 = 4sq. in. Substituting: 16v 1 =4 V2 or v2 = 4v I Therefore, the velocity of flow at point B is four times the velocity of flow at point A. VOLUME OF FLOW AND SPEED If you consider the cylinder volume you must fill and the distance the piston must travel, you can relate the volume of flow to the speed of the piston. The volume of the cylinder is found by multiplying the piston area by the length the piston must travel (stroke). Suppose you have determined that two cylinders have the same volume and that one cylinder is twice as long as the other. In this case, the cross-sectional area of the longer tube will be half of the cross-sectional area of the other tube. If fluid is pumped into each cylinder at the same rate, both pistons will reach their full travel at the same time. However, the piston in the smaller cylinder must travel twice as fast because it has twice as far to go. There are two ways of controlling the speed of the piston, (1) by varying the size of the cylinder and (2) by varying the volume of flow (gpm) to the cylinders. (Hydraulic cylinders are discussed in detail in chapter 10. ) STREAMLINE AND TURBULENT FLOW At low velocities or in tubes of small diameter, flow is streamlined. This means that a given particle of fluid moves straight forward without bumping into other particles and without crossing their paths. Streamline flow is often referred to as laminar flow, which is defined as a flow situation in which fluid moves in parallel lamina or layers. As an example of streamline flow, consider figure 2-14, which illustrates an open stream flowing at a slow, uniform rate with logs floating on its surface. The logs represent particles of fluid. As long as the stream flows at a slow, uniform rate, each log floats downstream in its Figure 2-14.—Streamline flow. own path, without crossing or bumping into the other. If the stream narrows, however, and the volume of flow remains the same, the velocity of flow increases. If the velocity increases sufficiently, the water becomes turbulent. (See fig. 2-15.) Swirls, eddies, and cross-motions are set up in the water. As this happens, the logs are thrown against each other and against the banks of the stream, and the paths followed by different logs will cross and recross. Particles of fluid flowing in pipes act in the same manner. The flow is streamlined if the fluid flows slowly enough, and remains streamlined at greater velocities if the diameter of the pipe is small. If the velocity of flow or size of pipe is increased sufficiently, the flow becomes turbulent. While a high velocity of flow will produce turbulence in any pipe, other factors contribute to turbulence. Among these are the roughness of the inside of the pipe, obstructions, the degree of curvature of bends, and the number of bends in the pipe. In setting up or maintaining fluid power systems, care should be taken to eliminate or Figure 2-15.—Turbulent flow. 2-10

minimize as many causes of turbulence as possible, since the energy consumed by turbulence is wasted. Limitations related to the degree and number of bends of pipe are discussed in chapter 5. While designers of fluid power equipment do what they can to minimize turbulence, it cannot be avoided. For example, in a 4-inch pipe at 68°F, flow becomes turbulent at velocities over approxi- mately 6 inches per second or about 3 inches per second in a 6-inch pipe. These velocities are far below those commonly encountered in fluid power systems, where velocities of 5 feet per second and above are common. In streamlined flow, losses due to friction increase directly with velocity. With turbulent flow these losses increase much more rapidly. FACTORS INVOLVED IN FLOW An understanding of the behavior of fluids in motion, or solids for that matter, requires an understanding of the term inertia. Inertia is the term used by scientists to describe the property possessed by all forms of matter that makes the matter resist being moved if it is at rest, and likewise, resist any change in its rate of motion if it is moving. The basic statement covering inertia is Newton’s first law of motion—inertia. Sir Isaac Newton was a British philosopher and mathe- matician. His first law states: A body at rest tends to remain at rest, and a body in motion tends to remain in motion at the same speed and direction, unless acted on by some unbalanced force. This simply says what you have learned by experience—that you must push an object to start it moving and push it in the opposite direction to stop it again. A familiar illustration is the effort a pitcher must exert to make a fast pitch and the opposition the catcher must put forth to stop the ball. Similarly, considerable work must be performed by the engine to make an automobile begin to roll; although, after it has attained a certain velocity, it will roll along the road at uniform speed if just enough effort is expended to overcome friction, while brakes are necessary to stop its motion. Inertia also explains the kick or recoil of guns and the tremendous striking force of projectiles. Inertia To and Force overcome the tendency of an object to resist any change in its state of rest or motion, some force that is not otherwise canceled or unbalanced must act on the object. Some unbalanced force must be applied whenever fluids are set in motion or increased in velocity; while conversely, forces are made to do work elsewhere whenever fluids in motion are retarded or stopped. There is a direct relationship between the magnitude of the force exerted and the inertia against which it acts. This force is dependent on two factors: (1) the mass of the object (which is proportional to its weight), and (2) the rate at which the velocity of the object is changed. The rule is that the force in pounds required to overcome inertia is equal to the weight of the object multiplied by the change in velocity, measured in feet per second, and divided by 32 times the time in seconds required to accomplish the change. Thus, the rate of change in velocity of an object is proportional to the force applied. The number 32 appears because it is the conversion factor between weight and mass. There are five physical factors that can act on a fluid to affect its behavior. All of the physical actions of fluids in all systems are determined by the relationships of these five factors to each other. Summarizing, these five factors are as follows: 1. Gravity, which acts at all times on all bodies, regardless of other forces 2. Atmospheric pressure, which acts on any part of a system exposed to the open air 3. Specific applied forces, which mayor may not be present, but which, in any event, are entirely independent of the presence or absence of motion 4. Inertia, which comes into play whenever there is a change from rest to motion or the opposite, or whenever there is a change in direction or in rate of motion 5. Friction, which is always present whenever there is motion 2-11

GRAVITY C INERTIA (VELOCITY HEAD) DB A ATMOSPHERIC PRESSURE APPLIED FORCE X Y X ATMOSPHERIC PRESSURE B FRICTION F P1 P FPf02016 Figure 2-16 illustrates a possible relationship of these factors with respect to a particle of fluid (P) in a system. The different forces are shown in terms of head, or in other words, in terms of vertical columns of fluid required to provide the forces. At the particular moment under consideration, a particle of water (P) is being acted on by applied force (A), by atmospheric pressure (B), and by gravity (C) produced by the weight of the fluid standing over it. The particle possesses sufficient inertia or velocity head to rise to level P1, since head equivalent to F was lost in friction as P passed through the system. Since atmospheric pressure (B) acts downward on both sides of the system, what is gained on one side is lost on the other. If all the pressure acting on P to force it through the nozzle could be recovered in the form of elevation head, it would rise to level Y. If account is taken of the balance in atmospheric pressure, in a frictionless system, P would rise to level X, or precisely as high as the sum of the gravity head and the head equivalent to the applied force. Kinetic Energy It was previously pointed out that a force must be applied to an object in order to give it a velocity or to increase the velocity it already has. Whether the force begins or changes velocity, it acts over a certain distance. A force acting over a certain distance is work. Work and all forms into which it can be changed are classified as energy. Obviously then, energy is required to give an object velocity. The greater the energy used, the greater the velocity will be. Disregarding friction, for an object to be brought to rest or for its motion to be slowed down, a force opposed to its motion must be applied to it. This force also acts over some distance. In this way energy is given up by the object and delivered in some form to whatever opposes its continuous motion. The moving object is therefore a means of receiving energy at one place (where its motion is increased) and delivering it to another point (where it is stopped or retarded). While it is in motion, it is said to contain this energy as energy of motion or kinetic energy. Since energy can never be destroyed, it follows that if friction is disregarded the energy delivered to stop the object will exactly equal the energy that was required to increase its speed. At all times the amount of kinetic energy possessed by an object depends on its weight and the velocity at which it is moving. Figure 2-16.—Physical factors governing fluid flow. 2-12

The mathematical relationship for kinetic energy is stated in the rule: “Kinetic energy in foot-pounds is equal to the force in pounds which created it, multiplied by the distance through which it was applied, or to the weight of the moving object in pounds, multiplied by the square of its velocity in feet per second, and divided by 64.s” The relationship between inertia forces, velocity, and kinetic energy can be illustrated by analyzing what happens when a gun fires a projectile against the armor of an enemy ship. (See fig. 2-17.) The explosive force of the powder in the breach pushes the projectile out of the gun, giving it a high velocity. Because of its inertia, the projectile offers opposition to this sudden velocity and a reaction is set up that pushes the gun backward (kick or recoil). The force of the explosion acts on the projectile throughout its movement in the gun. This is force acting through a distance producing work. This work appears as kinetic energy in the speeding projectile. The resistance of the air produces friction, which uses some of the energy and slows down the projectile. Eventually, however, the projectile hits its target and, because of the inertia, tries to continue moving. The target, being relatively stationary, tends to remain stationary because of its inertia. The result is that a tremendous force is set up that either leads to the penetration of the armor or the shattering of the projectile. The projectile is simply a means of transferring energy, in this instance for destructive purpose, from the gun to the enemy ship. This energy is transmitted in the form of energy of motion or kinetic energy. A similar action takes place in a fluid power system in which the fluid takes the place of the projectile. For example, the pump in a hydraulic Figure 2-17.—Relationship of inertia, velocity, and kinetic energy. system imparts energy to the fluid, which overcomes the inertia of the fluid at rest and causes it to flow through the lines. The fluid flows against some type of actuator that is at rest. The fluid tends to continue flowing, overcomes the inertia of the actuator, and moves the actuator to do work. Friction uses up a portion of the energy as the fluid flows through the lines and components. RELATIONSHIP OF FORCE, PRESSURE, AND HEAD In dealing with fluids, forces are usually considered in relation to the areas over which they are applied. As previously discussed, a force acting over a unit area is a pressure, and pressure can alternately be stated in pounds per square inch or in terms of head, which is the vertical height of the column of fluid whose weight would produce that pressure. In most of the applications of fluid power in the Navy, applied forces greatly outweigh all other forces, and the fluid is entirely confined. Under these circumstances it is customary to think of the forces involved in terms of pressures. Since the term head is encountered frequently in the study of fluid power, it is necessary to understand what it means and how it is related to pressure and force. All five of the factors that control the actions of fluids can, of course, be expressed either as force, or in terms of equivalent pressures or head. In each situation, the different factors are referred to in the same terms, since they can be added and subtracted to study their relationship to each other. At this point you need to review some terms in general use. Gravity head, when it is important enough to be considered, is sometimes referred to as head. The effect of atmospheric pressure is referred to as atmospheric pressure. (Atmospheric pressure is frequently and improperly referred to as suction.) Inertia effect, because it is always directly related to velocity, is usually called velocity head; and friction, because it represents a loss of pressure or head, is usually referred to as friction head. STATIC AND DYNAMIC FACTORS Gravity, applied forces, and atmospheric pressure are static factors that apply equally to 2-13

100 LBS. PER SQ. IN. 100 90 100 AB X FPf02018 fluids at rest or in motion, while inertia and friction are dynamic factors that apply only to fluids in motion. The mathematical sum of gravity, applied force, and atmospheric pressure is the static pressure obtained at any one point in a fluid at any given time. Static pressure exists in addition to any dynamic factors that may also be present at the same time. Remember, Pascal’s law states that a pressure set up in a fluid acts equally in all directions and at right angles to the containing surfaces. This covers the situation only for fluids at rest or practically at rest. It is true only for the factors making up static head. Obviously, when velocity becomes a factor it must have a direction, and as previously explained, the force related to the velocity must also have a direction, so that Pascal’s law alone does not apply to the dynamic factors of fluid power. The dynamic factors of inertia and friction are related to the static factors. Velocity head and friction head are obtained at the expense of static head. However, a portion of the velocity head can always be reconverted to static head. Force, which can be produced by pressure or head when dealing with fluids, is necessary to start a body moving if it is at rest, and is present in some form when the motion of the body is arrested; therefore, whenever a fluid is given velocity, some part of its original static head is used to impart this velocity, which then exists as velocity head. BERNOULLI’S PRINCIPLE Consider the system illustrated in figure 2-18. Chamber A is under pressure and is connected by a tube to chamber B, which is also under pressure. The pressure in chamber A is static pressure of 100 psi. The pressure at some point (X) along the connecting tube consists of a velocity pressure of Figure 2-18.—Relation of static and dynamic factors— Bernoulli’s principle. 10 psi exerted in a direction parallel to the line of flow, plus the unused static pressure of 90 psi, which still obeys Pascal’s law and operates equally in all directions. As the fluid enters chamber B it is slowed down, and its velocity is changed back to pressure. The force required to absorb its inertia equals the force required to start the fluid moving originally, so that the static pressure in chamber B is equal to that in chamber A. This situation (fig. 2-18) disregards friction; therefore, it would not be encountered in actual practice. Force or head is also required to overcome friction but, unlike inertia effect, this force cannot be recovered again, although the energy represented still exists somewhere as heat. Therefore, in an actual system the pressure in chamber B would be less than in chamber A by the amount of pressure used in overcoming friction along the way. At all points in a system the static pressure is always the original static pressure, less any velocity head at the point in question and less the friction head consumed in reaching that point. Since both the velocity head and the friction head represent energy that came from the original static head, and since energy cannot be destroyed, the sum of the static head, the velocity head, and the friction head at any point in the system must add up to the original static head. This is known as Bernoulli's principle, which states: For the horizontal flow of fluid through a tube, the sum of the pressure and the kinetic energy per unit volume of the fluid is constant. This principle governs the relations of the static and dynamic factors concerning fluids, while Pascal’s law states the manner in which the static factors behave when taken by themselves. MINIMIZING FRICTION Fluid power equipment is designed to reduce friction to the lowest possible level. Volume and velocity of flow are made the subject of careful study. The proper fluid for the system is chosen. Clean, smooth pipe of the best dimensions for the particular conditions is used, and it is installed along as direct a route as possible. Sharp bends and sudden changes in cross-sectional areas are avoided. Valves, gauges, and other components are designed to interrupt flow as little as possible. Careful thought is given to the size and shape of the openings. The systems are designed so they 2-14

1250 LBS. 25 LBS. OUTPUT PISTON 250 SQ. IN. INPUT PISTON 5 SQ. IN. 5 LBS. PER SQ. INCH. FPf02019 can be kept clean inside and variations from normal operation can easily be detected and remedied. OPERATION OF HYDRAULIC COMPONENTS To transmit and control power through pressurized fluids, an arrangement of inter- connected components is required. Such an arrangement is commonly referred to as a system. The number and arrangement of the components vary from system to system, depending on the particular application. In many applications, one main system supplies power to several subsystems, which are sometimes referred to as circuits. The complete system may be a small compact unit; more often, however, the components are located at widely separated points for convenient control and operation of the system. The basic components of a fluid power system are essentially the same, regardless of whether the system uses a hydraulic or a pneumatic medium. There are five basic components used in a system. These basic components are as follows: 1. 2. 3. 4. 5. Reservoir or receiver Pump or compressor Lines (pipe, tubing, or flexible hose) Directional control valve Actuating device Several applications of fluid power require only a simple system; that is, a system which uses only a few components in addition to the five basic components. A few of these applications are presented in the following paragraphs. We will explain the operation of these systems briefly at this time so you will know the purpose of each component and can better understand how hydraulics is used in the operation of these systems. More complex fluid power systems are described in chapter 12. HYDRAULIC JACK The hydraulic jack is perhaps one of the simplest forms of a fluid power system. By moving the handle of a small device, an individual can lift a load weighing several tons. A small initial force exerted on the handle is transmitted by a fluid to a much larger area. To understand this better, study figure 2-19. The small input piston has an area of 5 square inches and is directly connected to a large cylinder with an output piston having an area of 250 square inches. The top of this piston forms a lift platform. If a force of 25 pounds is applied to the input piston, it produces a pressure of 5 psi in the fluid, that is, of course, if a sufficient amount of resistant force is acting against the top of the output piston. Disregarding friction loss, this pressure acting on the 250 square inch area of the output piston will support a resistance force of 1,250 pounds. In other words, this pressure could overcome a force of slightly under 1,250 pounds. An input force of 25 pounds has been transformed into a working force of more than half a ton; however, for this to be true, the distance traveled by the input piston must be 50 times greater than the distance traveled by the output piston. Thus, for every inch that the input piston moves, the output piston will move only one-fiftieth of an inch. This would be ideal if the output piston needed to move only a short distance. However, in most instances, the output piston would have to be capable of moving a greater distance to serve a practical application. The device shown in figure 2-19 is not capable of moving the output piston farther than that shown; therefore, some other means must be used to raise the output piston to a greater height. Figure 2-19.—Hydraulic jack. 2-15

A INPUT PISTON 5 SQ. IN. OUTPUT PISTON 250 SQ. IN. CYLINDER 1 VALVE 1VALVE 3 5 LBS. PER SQ. INCH VALVE 2 RESERVOIR CYLINDER 2 B INPUT PISTON 5 SQ. IN. OUTPUT PISTON 250 SQ. IN. CYLINDER 1 VALVE 1VALVE 3 5 LBS. PER SQ. INCH VALVE 2 RESERVOIR CYLINDER 2 1250 LBS. 25 LBS. FPf02020 100 LBS. 1500 LBS. PISTON 10 SQ. IN. 10 LBS. PER SQ. IN. 10 LBS. PER SQ. IN. PISTON 3 300 SQ. IN. 3000 LBS. PISTON 2 150 SQ. IN. FPf02021 The output piston can be raised higher and maintained at this height if additional components are installed as shown in figure 2-20. In this illustration the jack is designed so that it can be raised, lowered, or held at a constant height. These results are attained by introducing a number of valves and also a reserve supply of fluid to be used in the system. Notice that this system contains the five basic components—the reservoir; cylinder 1, which serves as a pump; valve 3, which serves as a directional control valve; cylinder 2, which serves as the actuating device; and lines to transmit the fluid to and from the different components. In addition, this system contains two valves, 1 and 2, whose functions are explained in the following discussion. As the input piston is raised (fig. 2-20, view A), valve 1 is closed by the back pressure from the weight of the output piston. At the same time, valve 2 is opened by the head of the fluid in the reservoir. This forces fluid into cylinder 1. When the input piston is lowered (fig. 2-20, view B), a pressure is developed in cylinder 1. When this pressure exceeds the head in the reservoir, it closes valve 2. When it exceeds the back pressure from the output piston, it opens valve 1, forcing fluid into the pipeline. The pressure from cylinder 1 is Figure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke. thus transmitted into cylinder 2, where it acts to raise the output piston with its attached lift platform. When the input piston is again raised, the pressure in cylinder 1 drops below that in cylinder 2, causing valve 1 to close. This prevents the return of fluid and holds the output piston with its attached lift platform at its new level. During this stroke, valve 2 opens again allowing a new supply of fluid into cylinder 1 for the next power (downward) stroke of the input piston. Thus, by repeated strokes of the input piston, the lift platform can be progressively raised. To lower the lift platform, valve 3 is opened, and the fluid from cylinder 2 is returned to the reservoir. HYDRAULIC BRAKES The hydraulic brake system used in the automobile is a multiple piston system. A multiple piston system allows forces to be transmitted to two or more pistons in the manner indicated in figure 2-21. Note that the pressure set up by the force applied to the input piston (1) is transmitted undiminished to both output pistons (2 and 3), and that the resultant force on each piston is proportional to its area. The multiplication of forces from the input piston to each output piston is the same as that explained earlier. The hydraulic brake system from the master cylinders to the wheel cylinders on most Figure 2-21.—Multiple piston system. 2-16

WHEEL CYLINDERS FPf02022 BRAKE PEDAL MASTER CYLINDER PISTON WHEEL CYLINDER PISTONS FORCE automobiles operates in a way similar to the system illustrated in figure 2-22. When the brake pedal is depressed, the pressure on the brake pedal moves the piston within the master cylinder, forcing the brake fluid from the master cylinder through the tubing and flexible hose to the wheel cylinders. The wheel cylinders contain two opposed output pistons, each of which is attached to a brake shoe fitted inside the brake drum. Each output piston pushes the attached brake shoe against the wall of the brake drum, thus retarding the rotation of the wheel. When pressure on the pedal is released, the springs on the brake shoes return the wheel cylinder pistons to their released positions. This action forces the displaced brake fluid back through the flexible hose and tubing to the master cylinder. The force applied to the brake pedal produces a proportional force on each of the output pistons, which in turn apply the brake shoes frictionally to the turning wheels to retard rotation. As previously mentioned, the hydraulic brake system on most automobiles operates in a similar way, as shown in figure 2-22. It is beyond the scope of this manual to discuss the various brake systems. Figure 2-22.—An automobile brake system. 2-17

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CHAPTER 3 HYDRAULIC FLUIDS During the design of equipment that requires fluid power, many factors are considered in selecting the type of system to be used—hydraulic, pneumatic, or a combination of the two. Some of the factors are required speed and accuracy of operation, surrounding atmospheric conditions, economic conditions, availability of replacement fluid, required pressure level, operating tempera- ture range, contamination possibilities, cost of transmission lines, limitations of the equipment, lubricity, safety to the operators, and expected service life of the equipment. After the type of system has been selected, many of these same factors must be considered in selecting the fluid for the system. This chapter is devoted to hydraulic fluids. Included in it are sections on the properties and characteristics desired of hydraulic fluids; types of hydraulic fluids; hazards and safety precautions for working with, handling, and disposing of hydraulic liquids; types and control of contamination; and sampling. PROPERTIES If fluidity (the physical property of a substance that enables it to flow) and incompressibility were the only properties required, any liquid not too thick might be used in a hydraulic system. However, a satisfactory liquid for a particular system must possess a number of other properties. The most important properties and some charac- teristics are discussed in the following paragraphs. VISCOSITY Viscosity is one of the most important properties of hydraulic fluids. It is a measure of a fluid’s resistance to flow. A liquid, such as gasoline, which flows easily has a low viscosity; and a liquid, such as tar, which flows slowly has a high viscosity. The viscosity of a liquid is affected by changes in temperature and pressure. As the temperature of a liquid increases, its viscosity decreases. That is, a liquid flows more easily when it is hot than when it is cold. The viscosity of a liquid increases as the pressure on the liquid increases. A satisfactory liquid for a hydraulic system must be thick enough to give a good seal at pumps, motors, valves, and so on. These com- ponents depend on close fits for creating and maintaining pressure. Any internal leakage through these clearances results in loss of pressure, instantaneous control, and pump efficiency. Leakage losses are greater with thinner liquids (low viscosity). A liquid that is too thin will also allow rapid wearing of moving parts, or of parts that operate under heavy loads. On the other hand, if the liquid is too thick (viscosity too high), the internal friction of the liquid will cause an increase in the liquid’s flow resistance through clearances of closely fitted parts, lines, and internal passages. This results in pressure drops throughout the system, sluggish operation of the equipment, and an increase in power consumption. Measurement of Viscosity Viscosity is normally determined by measuring the time required for a fixed volume of a fluid (at a given temperature) to flow through a calibrated orifice or capillary tube. The instru- ments used to measure the viscosity of a liquid are known as viscometers or viscosimeters. Several types of viscosimeters are in use today. The Saybolt viscometer, shown in figure 3-1, measures the time required, in seconds, for 60 milliliters of the tested fluid at 100°F to pass through a standard orifice. The time measured is 3-1

INDICATOR DISK BEARING OPERATOR BODY BUSHINGS RETAINING NUT THRUST WASHERS GASKET RETAINING RING DISK PIN VALVE BODY BALL GASKET BALL SEATS BUMPER VALVE STEM BEARING RETAINER TAILPIECE BEARING INTERNAL GEAR GEAR RING GEAR BONNET GREASE PLUG ECCENTRIC SHAFT INDICATOR SHAFTINDICATOR HANDWHEEL FPf06002 same function as the disk in other valves. As the valve handle is turned to open the valve, the ball rotates to a point where part or all of the hole through the ball is in line with the valve body inlet and outlet, allowing fluid to flow through the valve. When the ball is rotated so the hole is perpendicular to the flow openings of the valve body, the flow of fluid stops. Most ball valves are the quick-acting type. They require only a 90-degree turn to either completely open or close the valve. However, many are operated by planetary gears. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves also contain a swing check located within the ball to give the valve a check valve feature. Figure 6-2 shows a ball-stop, swing-check valve with a planetary gear operation. In addition to the ball valves shown in figures 6-1 and 6-2, there are three-way ball valves that are used to supply fluid from a single source to one component or the other in a two-component system (fig. 6-3). Figure 6-2.—Typical ball-stop, swing-check valve. 6-2

FPf06005 FPf06004 OPEN CLOSED PORT A FPf06003 PORT B PORT C Figure 6-3.—Three-way ball valve. GATE VALVES Gate valves are used when a straight-line flow of fluid and minimum flow restriction are needed. Gate valves are so-named because the part that either stops or allows flow through the valve acts somewhat like a gate. The gate is usually wedge-shaped. When the valve is wide open the gate is fully drawn up into the valve bonnet. This leaves an opening for flow through the valve the same size as the pipe in which the valve is installed (fig. 6-4). Therefore, there is little pressure drop or flow restriction through the valve. Gate valves are not suitable for throttling purposes. The control of flow is difficult because of the valve’s design, and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling. Gate valves are classified as either rising-stem or nonrising-stem valves. The nonrising-stem valve is shown in figure 6-4. The stem is threaded into the gate. As the handwheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate. Valves with rising stems (fig. 6-5) are used when it is important to know by immediate inspection whether the valve is open or closed and when the threads (stem and gate) exposed to the fluid could become damaged by fluid contami- nants. In this valve, the stem rises out of the valve when the valve is opened. GLOBE VALVES Globe valves are probably the most common valves in existence. The globe valve gets its name Figure 6-4.—Operation of a gate valve. 6-3 Figure 6-5.—Rising stem gate valve.

VIEW A VIEW B FPf06007 Figure 6-6.—Types of globe valve bodies. from the globular shape of the valve body. Other types of valves may also have globular-shaped bodies. Thus, it is the internal structure of the valve that identifies the type of valve. The inlet and outlet openings for globe valves are arranged in a way to satisfy the flow requirements. Figure 6-6 shows straight-, angle-, and cross-flow valves. The moving parts of a globe valve consist of the disk, the valve stem, and the handwheel. The stem connects the handwheel and the disk. It is threaded and fits into the threads in the valve bonnet. The part of the globe valve that controls flow is the disk, which is attached to the valve stem. (Disks are available in various designs.) The valve is closed by turning the valve stem in until the disk is seated into the valve seat. This prevents fluid from flowing through the valve (fig. 6-7, view A). The edge of the disk and the seat are very accurately machined so that they forma tight seal when the valve is closed. When the valve is open (fig. 6-7, view B), the fluid flows through the space between the edge of the disk and the seat. Since the fluid flows equally on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the seat. The valve is commonly used as a fully open or fully closed valve, but it may be used as a throttle valve. However, since the seating surface is a relatively large area, it is not suitable as a throttle valve, where fine adjustments are required in controlling the rate of flow. The globe valve should never be jammed in the open position. After a valve is fully opened, the handwheel should be turned toward the closed position approximately one-half turn. Unless this is done, the valve is likely to seize in the open position, making it difficult, if not impossible, to close the valve. Many valves are damaged in this Figure 6-7.—Operation of a globe valve. 6-4 FPf06006 STRAIGHT-FLOW ANGLE-FLOW CROSS-FLOW Figure 6-6.—Types of globe valve bodies. from the globular shape of the valve body. Other types of valves may also have globular-shaped bodies. Thus, it is the internal structure of the valve that identifies the type of valve. The inlet and outlet openings for globe valves are arranged in a way to satisfy the flow requirements. Figure 6-6 shows straight-, angle-, and cross-flow valves. The moving parts of a globe valve consist of the disk, the valve stem, and the handwheel. The stem connects the handwheel and the disk. It is threaded and fits into the threads in the valve bonnet. The part of the globe valve that controls flow is the disk, which is attached to the valve stem. (Disks are available in various designs.) The valve is closed by turning the valve stem in until the disk is seated into the valve seat. This prevents fluid from flowing through the valve (fig. 6-7, view A). The edge of the disk and the seat are very accurately machined so that they forma tight seal when the valve is closed. When the valve is open (fig. 6-7, view B), the fluid flows through the space between the edge of the disk and the seat. Since the fluid flows equally on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the seat. The valve is commonly used as a fully open or fully closed valve, but it may be used as a throttle valve. However, since the seating surface is a relatively large area, it is not suitable as a throttle valve, where fine adjustments are required in controlling the rate of flow. The globe valve should never be jammed in the open position. After a valve is fully opened, the handwheel should be turned toward the closed position approximately one-half turn. Unless this is done, the valve is likely to seize in the open position, making it difficult, if not impossible, to close the valve. Many valves are damaged in this Figure 6-7.—Operation of a globe valve. 6-4

FPf06009 CLOSED FPf06008 OPEN manner. Another reason for not leaving globe valves in the fully open position is that it is sometimes difficult to determine if the valve is open or closed. If the valve is jammed in the open position, the stem may be damaged or broken by someone who thinks the valve is closed, and attempts to open it. It is important that globe valves be installed with the pressure against the face of the disk to keep the system pressure away from the stem packing when the valve is shut. NEEDLE VALVES Needle valves are similar in design and operation to the globe valve. Instead of a disk, a needle valve has a long tapered point at the end of the valve stem. A cross-sectional view of a needle valve is illustrated in figure 6-8. The long taper of the valve element permits a much smaller seating surface area than that of the globe valve; therefore, the needle valve is more suitable as a throttle valve. Needle valves are used to control flow into delicate gauges, which might be damaged by sudden surges of fluid under pressure. Needle valves are also used to control the end of a work cycle, where it is desirable for motion to be brought slowly to a halt, and at other points where precise adjustments of flow are necessary and where a small rate of flow is desired. Although many of the needle valves used in fluid power systems are the manually operated type (fig. 6-8), modifications of this type of valve are often used as variable restrictors. This valve is constructed without a handwheel and is adjusted to provide a specific rate of flow. This rate of flow will provide a desired time of operation for a particular subsystem. Since this type of valve can be adjusted to conform to the requirements of a particular system, it can be used in a variety of systems. Figure 6-9 illustrates a needle valve that was modified as a variable restrictor. HYDRAULIC AND PNEUMATIC GLOBE VALVES The valve consists of a valve body and a stem cartridge assembly. The stem cartridge assembly includes the bonnet, gland nut, packing, packing retainer, handle, stem, and seat. On small valves (1/8 and 1/4 inch) the stem is made in one piece, but on larger sizes it is made of a stem, guide, and stem retainer. The valve disk is made of nylon and is swaged into either the stem, for 1/8- and 1/4-inch valves, or the guide, for larger valves. The bonnet screws into the valve body with left-hand threads and is sealed by an O-ring (including a back-up ring). Figure 6-8. —Cross-sectional view of a needle valve. Figure 6-9. —Variable restrictor. 6-5

FPf06010 STEM HANDLE BONNET GUIDE DISK INSERT SEAT BODY SEAT O-RING O-RING BACK-UP RING PACKING RETAINER GLAND NUT BONNET STEM 1/8" 8 1/4" 3/8"-2" GLOBE VALVE The valve is available with either a rising stem or a non-rising stem. The rising stem valve uses the same port body design as does the non-rising stem valve. The stem is threaded into the gland nut and screws outward as the valve is opened. This valve does not incorporate provisions for tightening the stem packing nor replacing the packing while the valve is in service; therefore, complete valve disassembly is required for maintenance. Figure 6-10 illustrates a rising stem hydraulic and pneumatic globe valve. Additional information on this valve is available in Standard Navy Valves, NAVSHIPS 0948-012-5000. PRESSURE CONTROL VALVES The safe and efficient operation of fluid power systems, system components, and related equipment requires a means of controlling pressure. There are many types of automatic pressure control valves. Some of them merely provide an escape for pressure that exceeds a set pressure; some only reduce the pressure to a lower pressure system or subsystem; and some keep the pressure in a system within a required range. RELIEF VALVES Some fluid power systems, even when operat- ing normally, may temporarily develop excessive pressure; for example, when an unusually strong work resistance is encountered. Relief valves are used to control this excess pressure. Relief valves are automatic valves used on system lines and equipment to prevent over- pressurization. Most relief valves simply lift (open) at a preset pressure and reset (shut) when the pressure drops slightly below the lifting pressure. They do not maintain flow or pressure at a given amount, but prevent pressure from rising above a specific level when the system is temporarily overloaded. Main system relief valves are generally installed between the pump or pressure source and the first system isolation valve. The valve must be large enough to allow the full output of the hydraulic pump to be delivered back to the reservoir. In a pneumatic system, the relief valve controls excess pressure by discharging the excess gas to the atmosphere. Figure 6-10.—Hydraulic and pneumatic globe valve (rising stem). 6-6

B AB AA B FPf06012 Smaller relief valves, similar in design and operation to the main system relief valve, are often used in isolated parts of the system where a check valve or directional control valve prevents pressure from being relieved through the main system relief valve and where pressures must be relieved at a set point lower than that provided by the main system relief. These small relief valves are also used to relieve pressures caused by thermal expansion (see glossary) of the fluids. Figure 6-11 shows a typical relief valve. System pressure simply acts under the valve disk at the inlet to the valve. When the system pressure exceeds the force exerted by the valve spring, the valve disk lifts off of its seat, allowing some of the system fluid to escape through the valve outlet until the system pressure is reduced to just below the relief set point of the valve. All relief valves have an adjustment for increasing or decreasing the set relief pressure. Some relief valves are equipped with an adjusting screw for this purpose. This adjusting screw is usually covered with a cap, which must be removed before an adjustment can be made. Some type of locking device, such as a lock nut, is usually provided to prevent the adjustment from changing through vibration. Other types of relief valves are equipped with a handwheel for making adjustments to the valve. Either the adjusting screw or the handwheel is turned clockwise to increase the pressure at which the valve will open. In addition, most relief valves are also provided Figure 6-11 .—Relief valve. with an operating lever or some type of device to allow manual cycling or gagging the valve open for certain tasks. Various modifications of the relief valve shown in figure 6-11 are used to efficiently serve the requirements of some fluid power systems; however, this relief valve is unsatisfactory for some applications. To give you a better under- standing of the operation of relief valves, we will discuss some of the undesirable characteristics of this valve. A simple relief valve, such as the one illustrated in figure 6-11, with a suitable spring adjustment can be set so that it will open when the system pressure reaches a certain level, 500 psi for example. When the valve does open, the volume of flow to be handled may be greater than the capacity of the valve; therefore, pressure in the system may increase to several hundred psi above the set pressure before the valve brings the pressure under control. A simple relief valve will be effective under these conditions only if it is very large. In this case, it would operate stiffly and the valve element would chatter back and forth. In addition, the valve will not close until the system pressure decreases to a point somewhat below the opening pressure. The surface area of the valve element must be larger than that of the pressure opening if the valve is to seat satisfactorily as shown in figure 6-12. The pressure in the system acts on the valve element open to it. In each case in figure 6-12, the force exerted directly upward by system pressure when the valve is closed depends on the area (A) across the valve element where the element seats against the pressure tube. The moment the valve opens, however, the upward force exerted depends on the horizontal area (B) of the entire valve element, which is greater than area A. This causes an upward jump of the valve element immediately after it opens, because the Figure 6-12.—Pressure acting on different areas. 6-7

FPf06013 VALVE CHAMBER PISTON PASSAGE STEM PASSAGE BALL PILOT VALVE SPRING PILOT VALVE ADJUSTING SCREW MAIN VALVE SPRING PISTON TO RESERVOIR STEM HIGH-PRESSURE LINE A B C same pressure acting over different areas produces forces proportional to the areas. It also requires a greater force to close the valve than was required to open it. As a result, the valve will not close until the system pressure has decreased to a certain point below the pressure required to open it. Let us assume that a valve of this type is set to open at 500 psi. (Refer to fig. 6-12.) When the valve is closed, the pressure acts on area A. If this area is 0.5 square inch, an upward force of 250 pounds (500 0.5) will be exerted on the valve at the moment of opening. With the valve open, however, the pressure acts on area B. If area B is 1 square inch, the upward force is 500 pounds, or double the force at which the valve actually opened. For the valve to close, pressure in the system would have to decrease well below the point at which the valve opened. The exact pressure would depend on the shape of the valve element. In some hydraulic systems, there is a pressure in the return line. This back pressure is caused by restrictions in the return line and will vary in relation to the amount of fluid flowing in the return line. This pressure creates a force on the back of the valve element and will increase the force necessary to open the valve and relieve system pressure. It follows that simple relief valves have a tendency to open and close rapidly as they “hunt” above and below the set pressure, causing pressure pulsations and undesirable vibrations and producing a noisy chatter. Because of the unsatisfactory performance of the simple relief valve in some applications, compound relief valves were developed. Compound relief valves use the principles of operation of simple relief valves for one stage of their action—that of the pilot valve. Provision is made to limit the amount of fluid that the pilot valve must handle, and thereby avoid the weaknesses of simple relief valves. (A pilot valve is a small valve used for operating another valve.) The operation of a compound relief valve is illustrated in figure 6-13. In view A, the main valve, which consists of a piston, stem, and spring, is closed, blocking flow from the high-pressure line to the reservoir. Fluid in the high-pressure line flows around the stem of the main valves as it flows to the actuating unit. The stem of the main valve is hollow (the stem passage) and contains the main valve spring, which forces the main valve against its seat. When the pilot valve is open the stem passage allows fluid to flow from the pilot Figure 6-13.—Operation of compound relief valve, 6-8

valve, around the main valve spring, and down to the return line. There is also a narrow passage (piston passage) through the main valve piston. This passage connects the high-pressure line to the valve chamber. The pilot valve is a small, ball-type, spring- loaded check valve, which connects the top of the passage from the valve chamber with the passage through the main valve stem. The pilot valve is the control unit of the relief valve because the pressure at which the relief valve will open depends on the tension of the pilot valve spring. The pilot valve spring tension is adjusted by turning the adjusting screw so that the ball will unseat when system pressure reaches the preset limit. Fluid at line pressure flows through the narrow piston passage to fill the chamber. Because the line and the chamber are connected, the pressure in both are equal. The top and bottom of the main piston have equal areas; therefore, the hydraulic forces acting upward and downward are equal, and there is no tendency for the piston to move in either direction. The only other force acting on the main valve is that of the main valve spring, which holds it closed. When the pressure in the high-pressure line increases to the point at which the pilot valve is set, the ball unseats (fig. 6-13, view B). This opens the valve chamber through the valve stem passage to the low-pressure return line. Fluid immediately begins to flow out of the chamber, much faster than it can flow through the narrow piston passage. As a result the chamber pressure immediately drops, and the pilot valve begins to close again, restricting the outward flow of fluid. Chamber pressure therefore increases, the valve opens, and the cycle repeats. So far, the only part of the valve that has moved appreciably is the pilot, which functions just like any other simple spring-loaded relief valve. Because of the small size of the piston passage, there is a severe limit on the amount of overpressure protection the pilot can provide the system. All the pilot valve can do is limit fluid pressure in the valve chamber above the main piston to a preset maximum pressure, by allowing excess fluid to flow through the piston passage, through the stem passage, and into the return line. When pressure in the system increases to a value that is above the flow capacity of the pilot valve, the main valve opens, permitting excess fluid to flow directly to the return line. This is accomplished in the following manner. As system pressure increases, the upward force on the main piston overcomes the downward force, which consists of the tension of the main piston spring and the pressure of the fluid in the valve chamber (fig. 6-13, view C). The piston then rises, unseating the stem, and allows the fluid to flow from the system pressure line directly into the return line. This causes system pressure to decrease rapidly, since the main valve is designed to handle the complete output of the pump. When the pressure returns to normal, the pilot spring forces the ball onto the seat. Pressures are equal above and below the main piston, and the main spring forces the valve to seat. As you can see, the compound valve over- comes the greatest limitation of a simple relief valve by limiting the flow through the pilot valve to the quantity it can satisfactorily handle. This limits the pressure above the main valve and enables the main line pressure to open the main valve. In this way, the system is relieved when an overload exists. PRESSURE REGULATORS Pressure regulators, often referred to as unloading valves, are used in fluid power systems to regulate pressure. In pneumatic systems, the valve, commonly referred to as a pressure regulator, simply reduces pressure. This type of valve is discussed later in this chapter under pressure-reducing valves. In hydraulic systems the pressure regulator is used to unload the pump and to maintain and regulate system pressure at the desired values. All hydraulic systems do not require pressure regulators. The open-center system (discussed in chapter 12) does not require a pressure regulator. Many systems are equipped with variable-displacement pumps (discussed in chapter 4), which contain a pressure-regulating device. Pressure regulators are made in a variety of types and by various manufacturers; however, the 6-9

RETURN TO RESERVOIR PILOT VALVE INLET OUTLET CHECK VALVE PISTON SPRING A B FPf06014 basic operating principles of all regulators are similar to the one illustrated in figure 6-14. A regulator is open when it is directing fluid under pressure into the system (fig. 6-14, view A). In the closed position (fig. 6-14, view B), the fluid in the part of the system beyond the regulator is trapped at the desired pressure, and the fluid from the pump is bypassed into the return line and back to the reservoir. To prevent constant opening and closing (chatter), the regulator is designed to open at a pressure somewhat lower than the closing pressure. This difference is known as differential or operating range. For example, assume that a pressure regulator is set to open when the system pressure drops below 600 psi, and close when the pressure rises above 800 psi. The differential or operating range is 200 psi. Referring to figure 6-14, assume that the piston has an area of 1 square inch, the pilot valve has a cross-sectional area of one-fourth square inch, and the piston spring provides 600 pounds of force pushing the piston down. When the pressure in the system is less than 600 psi, fluid from the pump will enter the inlet port, flow to the top of the regulator, and then to the pilot valve. When the pressure of the fluid at the inlet increases to the point where the force it creates against the front of the check valve exceeds the force created against the back of the check valve by system pressure and the check valve spring, the check valve opens. This allows fluid to flow into the system and to the bottom of the regulator against the piston. When the force created by the system pressure exceeds the force exerted by the spring, the piston moves up, causing the pilot valve to unseat. Since the fluid will take the path of least resistance, it will pass through the regulator and back to the reservoir through the return line. When the fluid from the pump is suddenly allowed a free path to return, the pressure on the input side of the check valve drops and the check valve closes. The fluid in the system is then trapped under pressure. This fluid will remain pressurized until a power unit is actuated, or until pressure is slowly lost through normal internal leakage within the system. When the system pressure decreases to a point slightly below 600 psi, the spring forces the piston down and closes the pilot valve. When the pilot valve is closed, the fluid cannot flow directly to the return line. This causes the pressure to increase in the line between the pump and the regulator. This pressure opens the check valve, causing the fluid to enter the system. In summary, when the system pressure decreases a certain amount, the pressure regulator will open, sending fluid to the system. When the system pressure increases sufficiently, the regulator will close, allowing the fluid from the pump to flow through the regulator and back to the reservoir. The pressure regulator takes the load off of the pump and regulates system pressure. Figure 6-14.—Hydraulic pressure regulator. 6-10

SEQUENCE VALVES FLUID SUPPLY TO RETURN CYLINDER A 1 2 CYLINDER B CYLINDER C FPf06015 Figure 6-15 .—Installation SEQUENCE VALVES of sequence valves. Sequence valves control the sequence of operation between two branches in a circuit; that is, they enable one unit to automatically set another unit into motion. An example of the use of a sequence valve is in an aircraft landing gear actuating system. In a landing gear actuating system, the landing gear doors must open before the landing gear starts to extend. Conversely, the landing gear must be completely retracted before the doors close. A sequence valve installed in each landing gear actuating line performs this function. A sequence valve is somewhat similar to a relief valve except that, after the set pressure has been reached, the sequence valve diverts the fluid to a second actuator or motor to do work in another part of the system. Figure 6-15 shows an installation of two sequence valves that control the sequence of operation of three actuating cylinders. Fluid is free to flow into cylinder A. The first sequence valve (1) blocks the passage of fluid until the piston in cylinder A moves to the end of its stroke. At this time, sequence valve 1 opens, allowing fluid to enter cylinder B. This action continues until all three pistons complete their strokes. There are various types of sequence valves. Some are controlled by pressure and some are controlled mechanically. Pressure-Controlled Sequence Valve The operation of a typical pressure-controlled sequence valve is illustrated in figure 6-16. The opening pressure is obtained by adjusting the tension of the spring that normally holds the piston in the closed position. (Note that the top part of the piston has a larger diameter than the lower part.) Fluid enters the valve through the inlet port, flows around the lower part of the piston and exits the outlet port, where it flows to the primary (first) unit to be operated (fig. 6-16, view A). This fluid pressure also acts against the lower surface of the piston. Figure 6-16.—Operation of a pressure-controlled sequence valve. 6-11 SPRING PISTON INLET A B ADJUSTING SCREW DRAIN OUTLET TO PRIMARY UNIT OUTLET TO SECONDARY UNIT FPf06016

When the primary actuating unit completes its operation, pressure in the line to the actuating unit increases sufficiently to overcome the force of the spring, and the piston rises. The valve is then in the open position (fig. 6-16, view B). The fluid entering the valve takes the path of least resistance and flows to the secondary unit. A drain passage is provided to allow any fluid leaking past the piston to flow from the top of the valve. In hydraulic systems, this drain line is usually connected to the main return line. Mechanically Operated Sequence Valve The mechanically operated sequence valve (fig. 6-17) is operated by a plunger that extends through the body of the valve. The valve is mounted so that the plunger will be operated by the primary unit. A check valve, either a ball or a poppet, is installed between the fluid ports in the body. It can be unseated by either the plunger or fluid pressure. Port A (fig. 6-17) and the actuator of the primary unit are connected by a common line. Port B is connected by a line to the actuator of the secondary unit. When fluid under pressure flows to the primary unit, it also flows into the sequence valve through port A to the seated check valve in the sequence valve. In order to operate the secondary unit, the fluid must flow through the sequence valve. The valve is located so that the primary unit depresses the plunger as it completes its operation. The plunger unseats the check valve and allows the fluid to flow Figure 6-17.—Mechanically operated sequence valve. through the valve, out port B, and to the secondary unit. This type of sequence valve permits flow in the opposite direction. Fluid enters port B and flows to the check valve. Although this is return flow from the actuating unit, the fluid overcomes spring tension, unseats the check valve, and flows out through port A. PRESSURE-REDUCING VALVES Pressure-reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system. A reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the valve is set, the reduced pressure will be maintained regardless of changes in supply pressure (as long as the supply pressure is at least as high as the reduced pressure desired) and regardless of the system load, providing the load does not exceed the design capacity of the reducer. Figure 6-18.—Spring-loaded pressure-reducing valve. 6-12 CHECK-VALVE ASSEMBLY PORT A PLUNGER PORT B PORT A FPf06017 PORT B HOUSING DIAPHRAGM INLET PORT (LINE PRESSURE) VALVE DISK OUTLET PORT (REDUCED PRESSURE) VALVE STEM ADJUSTING SPRING ADJUSTING SCREW FPf06018

There are various designs and types of pressure-reducing valves. The spring-loaded reducer and the pilot-controlled valve are discussed in this text. Spring-Loaded Reducer The spring-loaded pressure-reducing valve (fig. 6-18) is commonly used in pneumatic systems. It is often referred to as a pressure regulator. The valve simply uses spring pressure against a diaphragm to open the valve. On the bottom of the diaphragm, the outlet pressure (the pressure in the reduced-pressure system) of the valve forces the diaphragm upward to shut the valve. When the outlet pressure drops below the set point of the valve, the spring pressure overcomes the outlet pressure and forces the valve stem downward, opening the valve. As the outlet pressure increases, approaching the desired value, the pressure under the diaphragm begins to overcome spring pressure, forcing the valve stem upwards, shutting the valve. You can adjust the downstream pressure by turning the adjusting screw, which varies the spring pressure against the diaphragm. This particular spring-loaded valve will fail in the open position if a diaphragm rupture occurs. Pilot-Controlled Pressure-Reducing Valve Figure 6-19 illustrates the operation of a pilot-controlled pressure-reducing valve. This valve consists of an adjustable pilot valve, which controls the operating pressure of the valve, and a spool valve, which reacts to the action of the pilot valve. The pilot valve consists of a poppet (1), a spring (2), and an adjusting screw (3). The valve Figure 6-19.—Pilot-controlled pressure-reducing valve. 6-13 15 1. POPPET VALVE 2. PILOT VALVE SPRING 3. ADJUSTING SCREW 4. SPOOL VALVE SPRING 5. HIGH-PRESSURE OUTLET PORT 14 13 12 11 5 10 9 8 6 9 12 1 15 2 3 4 7 8 REDUCED PRESSURE OUTLET PORT OPENING FLUID PASSAGE FLUID PASSAGE VALVE SPOOL 6. 7. 8. 9. 10. HIGH-PRESSURE INLET PORT FLUID CHAMBER FLUID PASSAGE FLUID CHAMBER DRAIN 11. 12. 13. 14. 15. AB FPf06019 6

spool assembly consists of a valve spool (10) and a spring (4). Fluid under main pressure enters the inlet port (11) and under all conditions is free to flow through the valve and the outlet port (5). (Either port 5 or port 11 maybe used as the high-pressure port.) Figure 6-19, view A, shows the valve in the open position. In this position, the pressure in the reduced-pressure outlet port (6) has not reached the preset operating pressure of the valve. The fluid also flows through passage 8, through smaller passage 9 in the center of the valve spool, and into chamber 12. The fluid pressure at outlet port 6 is therefore distributed to both ends of the spool. When these pressures are equal the spool is hydrau- lically balanced. Spring 4 is a low-tension spring and applies only a slight downward force on the spool. Its main purpose is to position the spool and to maintain opening 7 at its maximum size. As the pressure increases in outlet port 6 (fig. 16, view B), this pressure is transmitted through passages 8 and 9 to chamber 12. This pressure also acts on the pilot valve poppet (1). When this pressure increases above the preset operating pressure of the valve, it overcomes the force of pilot valve spring 2 and unseats the poppet. This allows fluid to flow through the drain port (15). Because the small passage (9) restricts flow into chamber 12, the fluid pressure in the chamber drops. This causes a momentary difference in pressure across the valve spool (10) which allows fluid pressure acting against the bottom area of the valve spool to overcome the downward force of spring 4. The spool is then forced upward until the pressures across its ends are equalized. As the spool moves upward, it restricts the flow through opening 7 and causes the pressure to decrease in the reduced pressure outlet port 6. If the pressure in the outlet port continues to increase to a value above the preset pressure, the pilot valve will open again and the cycle will repeat. This allows the spool valve to move up higher into chamber 12; thus further reducing the size of opening 7. These cycles repeat until the desired pressure is maintained in outlet 6. When the pressure in outlet 6 decreases to a value below the preset pressure, spring 4 forces the spool downward, allowing more fluid to flow through opening 7. COUNTERBALANCE VALVE The counterbalance valve is normally located in the line between a directional control valve and the outlet of a vertically mounted actuating cylinder which supports weight or must be held 6-14 in position for a period of time. This valve serves as a hydraulic resistance to the actuating cylinder. For example, counterbalance valves are used in some hydraulically operated forklifts. The valve offers a resistance to the flow from the actuating cylinder when the fork is lowered. It also helps to support the fork in the UP position. Counterbalance valves are also used in air- launched weapons loaders. In this case the valve is located in the top of the lift cylinder. The valve requires a specific pressure to lower the load. If adequate pressure is not available, the load cannot be lowered. This prevents collapse of the load due to any malfunction of the hydraulic system. One type of counterbalance valve is illustrated in figure 6-20. The valve element is a balanced spool (4). The spool consists of two pistons permanently fixed on either end of a shaft. The inner surface areas of the pistons are equal; therefore, pressure acts equally on both areas regardless of the position of the valve and has no effect on the movement of the valve—hence, the term balanced. The shaft area between the two pistons provides the area for the fluid to flow Figure 6-20. —Counterbalance valve. 2 FPf06020 1 8 7 9 6 5 4 3 1. ADJUSTMENT SCREW 2. INTERNAL DRAIN 3. SPRING 4. SPOOL 5. PRESSURE INLET OR REVERSE FREE FLOW OUTLET 6. PILOT PASSAGE 7. CHECK VALVE 8. DISCHARGE OUTLET OR REVERSE FREE FLOW INLET 9. PISTON

when the valve is open. A small piston (9) is attached to the bottom of the spool valve. When the valve is in the closed position, the top piston of the spool valve blocks the discharge port (8). With the valve in this position, fluid flowing from the actuating unit enters the inlet port (5). The fluid cannot flow through the valve because discharge port 8 is blocked. However, fluid will flow through the pilot passage (6) to the small pilot piston. As the pressure increases, it acts on the pilot piston until it overcomes the preset pressure of spring 3. This forces the valve spool (4) up and allows the fluid to flow around the shaft of the valve spool and out discharge port 8. Figure 6-20 shows the valve in this position. During reverse flow, the fluid enters port 8. The spring (3) forces valve spool 4 to the closed position. The fluid pressure overcomes the spring tension of the check valve (7). The check valve opens and allows free flow around the shaft of the valve spool and out through port 5. The operating pressure of the valve can be adjusted by turning the adjustment screw (1), which increases or decreases the tension of the spring. This adjustment depends on the weight that the valve must support. It is normal for a small amount of fluid to leak around the top piston of the spool valve and into the area around the spring. An accumulation would cause additional pressure on top of the spool valve. This would require additional pressure to open the valve. The drain (2) provides a passage for this fluid to flow to port 8. DIRECTIONAL CONTROL VALVES Directional control valves are designed to direct the flow of fluid, at the desired time, to the point in a fluid power system where it will do work. The driving of a ram back and forth in its cylinder is an example of when a directional control valve is used. Various other terms are used to identify directional valves, such as selector valve, transfer valve, and control valve. This manual will use the term directional control valve to identify these valves. Directional control valves for hydraulic and pneumatic systems are similar in design and operation. However, there is one major difference. The return port of a hydraulic valve is ported through a return line to the reservoir, while the similar port of a pneumatic valve, commonly referred to as the exhaust port, is usually vented to the atmosphere. Any other differences are pointed out in the discussion of the valves. Directional control valves may be operated by differences in pressure acting on opposite sides of the valving element, or they maybe positioned manually, mechanically, or electrically. Often two or more methods of operating the same valve will be used in different phases of its action. CLASSIFICATION Directional control valves may be classified in several ways. Some of the different ways are by the type of control, the number of ports in the valve housing, and the specific function of the valve. The most common method is by the type of valving element used in the construction of the valve. The most common types of valving elements are the ball, cone or sleeve, poppet, rotary spool, and sliding spool. The basic operating principles of the poppet, rotary spool, and sliding spool valving elements are discussed in this text. Poppet The poppet fits into the center bore of the seat (fig. 6-21). The seating surfaces of the poppet and the seat are lapped or closely machined so that the center bore will be sealed when the poppet is Figure 6-21.—Operation of a simple poppet valve. 6-15 OPEN CLOSED INLET OUTLET INLET OUTLET FPf06021

seated (shut). The action of the poppet is similar to that of the valves in an automobile engine. In most valves the poppet is held in the seated position by a spring. The valve consists primarily of a movable poppet which closes against the valve seat. In the closed position, fluid pressure on the inlet side tends to hold the valve tightly closed. A small amount of movement from a force applied to the top of the poppet stem opens the poppet and allows fluid to flow through the valve. The use of the poppet as a-valving element is not limited to directional control valves. Rotary Spool The rotary spool directional control valve (fig. 6-22) has a round core with one or more passages or recesses in it. The core is mounted within a stationary sleeve. As the core is rotated within the stationary sleeve, the passages or recesses connect or block the ports in the sleeve. The ports in the sleeve are connected to the appropriate lines of the fluid system. Sliding spool The operation of a simple sliding spool directional control valve is shown in figure 6-23. The valve is so-named because of the shape of the valving element that slides back and forth to block and uncover ports in the housing. (The sliding element is also referred to as a piston.) The inner piston areas (lands) are equal. Thus fluid under pressure which enters the valve from the inlet ports CHECK VALVE Figure 6-22.—Parts of a rotary spool directional control valve. Figure 6-23.—Two-way, sliding spool directional control valve. acts equally on both inner piston areas regardless of the position of the spool. Sealing is usually accomplished by a very closely machined fit between the spool and the valve body or sleeve. For valves with more ports, the spool is designed with more pistons or lands on a common shaft. The sliding spool is the most commonly used type of valving element used in directional control valves. Check valves are used in fluid systems to permit flow in one direction and to prevent flow in the other direction. They are classified as one-way directional control valves. The check valve may be installed inde- pendently in a line to allow flow in one direction only, or it may be used as an integral part of globe, sequence, counterbalance, and pressure- reducing valves. Check valves are available in various designs. They are opened by the force of fluid in motion flowing in one direction, and are closed by fluid attempting to flow in the opposite direction. The force of gravity or the action of a spring aids in closing the valve. 6-16 PASSAGES FIXED SLEEVE CORE PORTS FPf06022 OUT OPEN INRESERVOIR RESERVOIR INRESERVOIR RESERVOIR OUT CLOSED FPf06023

Figure 6-24.—Swing check valve. Figure 6-24 shows a swing check valve. In the open position, the flow of fluid forces the hinged disk up and allows free flow through the valve. Flow in the opposite direction with the aid of gravity, forces the hinged disk to close the passage and blocks the flow. This type of valve is sometimes designed with a spring to assist in closing the valve. The most common type of check valve, installed in fluid-power systems, uses either a ball or cone for the sealing element (fig. 6-25). As fluid pressure is applied in the direction of the arrow, the cone (view A) or ball (view B) is forced off its seat, allowing fluid to flow freely through the valve. This valve is known as a spring-loaded check valve. The spring is installed in the valve to hold the cone or ball on its seat whenever fluid is not flowing. The spring also helps to force the cone or ball on its seat when the fluid attempts to flow in the opposite direction. Since the opening and closing of this type of valve is not dependent on gravity, its location in a system is not limited to the vertical position. A modification of the spring-loaded check valve is the orifice check valve (fig. 6-26). This Figure 6-25.—Spring-loaded check valves. Figure 6-26.—Typical orifice check valves. 6-17

valve allows normal flow in one direction and restricted flow in the other. It is often referred to as a one-way restrictor. Figure 6-26, view A, shows a cone-type orifice check valve. When sufficient fluid pressure is applied at the inlet port, it overcomes spring tension and moves the cone off of its seat. The two orifices (2) in the illustration represent several openings located around the slanted circumference of the cone. These orifices allow free flow of fluid through the valve while the cone is off of its seat. When fluid pressure is applied through the outlet port, the force of the fluid and spring tension move the cone to the left and onto its seat. This action blocks the flow of fluid through the valve, except through the orifice (1) in the center of the cone. The size of the orifice (in the center of the cone) determines the rate of flow through the valve as the fluid flows from right to left. Figure 6-26, view B, shows a ball-type orifice check valve. Fluid flow through the valve from left to right forces the ball off of its seat and allows normal flow. Fluid flow through the valve in the opposite direction forces the ball onto its seat. Thus, the flow is restricted by the size of the orifice located in the housing of the valve. NOTE: The direction of free flow through the orifice check valve is indicated by an arrow stamped on the housing. SHUTTLE VALVE In certain fluid power systems, the supply of fluid to a subsystem must be from more than one source to meet system requirements. In some systems an emergency system is provided as a source of pressure in the event of normal system failure. The emergency system will usually actuate only essential components. The main purpose of the shuttle valve is to isolate the normal system from an alternate or emergency system. It is small and simple; yet, it is a very important component. Figure 6-27 is a cutaway view of a typical shuttle valve. The housing contains three ports— normal system inlet, alternate or emergency system inlet, and outlet. A shuttle valve used to operate more than one actuating unit may contain additional unit outlet ports. Enclosed in the housing is a sliding part called the shuttle. Its purpose is to seal off either one or the other inlet ports. There is a shuttle seat at each inlet port. 6-18 Figure 6-27.—Shuttle valve. When a shuttle valve is in the normal operation position, fluid has a free flow from the normal system inlet port, through the valve, and out through the outlet port to the actuating unit. The shuttle is seated against the alternate system inlet port and held there by normal system pressure and by the shuttle valve spring. The shuttle remains in this position until the alternate system is activated. This action directs fluid under pressure from the alternate system to the shuttle valve and forces the shuttle from the alternate system inlet port to the normal system inlet port. Fluid from the alternate system then has a free flow to the outlet port, but is prevented from entering the normal system by the shuttle, which seals off the normal system port. The shuttle may be one of four types: (1) sliding plunger, (2) spring-loaded piston, (3) spring-loaded ball, or (4) spring-loaded poppet. In shuttle valves that are designed with a spring, the shuttle is normally held against the alternate system inlet port by the spring. TWO-WAY VALVES The term two-way indicates that the valve contains and controls two functional flow control ports-an inlet and an outlet. A two-way, sliding spool directional control valve is shown in figure 6-23. As the spool is moved back and forth, it either allows fluid to flow through the valve or prevents flow. In the open position, the fluid enters the inlet port, flows around the shaft of the spool, and through the outlet port. The spool cannot move back and forth by difference of

forces set up within the cylinder, since the forces there are equal. As indicated by the arrows against the pistons of the spool, the same pressure acts on equal areas on their inside surfaces. In the closed position, one of the pistons of the spool simply blocks the inlet port, thus preventing flow through the valve. A number of features common to most sliding spool valves are shown in figure 6-23. The small ports at either end of the valve housing provide a path for any fluid that leaks past the spool to flow to the reservoir. This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When spool valves become worn, they may lose balance because of greater leakage on one side of the spool than on the other. In that event, the spool would tend to stick when it is moved back and forth. Small grooves are therefore machined around the sliding surface of the piston; and in hydraulic valves, leaking liquid will encircle the pistons and keep the contacting surfaces lubricated and centered. THREE-WAY VALVES Three-way valves contain a pressure port, a cylinder port, and a return or exhaust port. The three-way directional control valve is designed to operate an actuating unit in one direction; it permits either the load on the actuating unit or a spring to return the unit to its original position. Cam-Operated Three-Way Valves Figure 6-28 shows the operation of a cam- operated, three-way, poppet-type directional control valve. View A shows fluid under pressure forcing the piston outward against a load. The upper poppet (2) is unseated by the inside cam (5), permitting fluid to flow from the line (3) into the cylinder to actuate the piston. The lower poppet (1) is seated, sealing off the flow into the return line (4). As the force of the pressurized fluid extends the piston rod, it also compresses the spring in the cylinder. View B shows the valve with the control handle turned to the opposite position. In this position, the upper poppet (2) is seated, blocking the flow of fluid from the pressure line (3). The lower poppet (1) is unseated by the outside cam (6). This releases the pressure in the cylinder and allows the spring to expand, which forces the piston rod to retract. The fluid from the cylinder flows through the control valve and out the return Figure 6-28.—Three-way, poppet-type directional control valve (cam-operated). 6-19 CONTROL HANDLE 3 4 6 2 1 5 A 3 4 6 2 1 5 B 1. LOWER (RETURN OR EXHAUST) POPPET 2. UPPER (PRESSURE) POPPET 3. PRESSURE LINE 4. RETURN OR EXHAUST PORT 5. INSIDE CAM 6. OUTSIDE CAM FPf06028

port (4). In hydraulic systems, the return port is connected by a line to the reservoir. In pneumatic systems, the return port is usually open to the atmosphere. Pilot-Operated Three-Way Valves A pilot-operated, poppet-type, three-way directional control valve is shown in figure 6-29. Valves of this design are often used in pneumatic systems. This valve is normally closed and is forced open by fluid pressure entering the pilot chamber. The valve contains two poppets connected to each other by a common stem. The poppets are connected to diaphragms which hold them in a centered position. The movement of the poppet is controlled by the pressure in the pilot port and the chamber above the upper diaphragm. When the pilot chamber is not pressurized, the lower poppet is seated against the lower valve seat. Fluid can flow from the supply line through the inlet port and through the holes in the lower diaphragm to fill the bottom chamber. This pressure holds the lower poppet tightly against its seat and blocks flow from the inlet port through the valve. At the same time, due to the common stem, the upper poppet is forced off of its seat. Fluid from the actuating unit flows through the open passage, around the stem, and through the exhaust port to the atmosphere. When the pilot chamber is pressurized, the force acting against the diaphragm forces the poppet down. The upper poppet closes against its seat, blocking the flow of fluid from the cylinder to the exhaust port. The lower poppet opens, and the passage from the supply inlet port to the cylinder port is open so that the fluid can flow to the actuating unit. The valve in figure 6-29 is a normally closed valve. Normally open valves are similar in design. When no pressure is applied to the pilot chamber, the upper poppet is forced off of its seat and the lower poppet is closed. Fluid is free to flow from the inlet port through the cylinder to the actuating unit. When pilot pressure is applied, the poppets are forced downward, closing the upper poppet and opening the lower poppet. Fluid can now flow from the cylinder through the valve and out the exhaust port to the atmosphere. FOUR-WAY VALVES Most actuating devices require system pressure for operation in either direction. The four-way directional control valve, which contains four ports, is used to control the operation of such devices. The four-way valve is also used in some systems to control the operation of other valves. It is one of the most widely used directional control valves in fluid power systems. The typical four-way directional control valve has four ports: a pressure port, a return or exhaust port, and two cylinder or working ports. The pressure port is connected to the main system pressure line and the return line is connected to the reservoir in hydraulic systems. In pneumatic systems the return port is usually vented to the atmosphere. The two cylinder ports are connected by lines to the actuating units. Poppet-Type Four-Way Valves Figure 6-30 shows atypical four-way, poppet- type directional control valve. This is a manually operated valve and consists of a group of conventional spring-loaded poppets. The poppets are enclosed in a common housing and are interconnected by ducts to direct the flow of fluid in the desired direction. Figure 6-29.—Three-way, poppet-type, normally closed directional control valve (pilot-operated). 6-20 EXHAUST FPf06029 OPEN SUPPLY CLOSEDCYLINDER CYLINDER EXHAUST CLOSED OPEN TO CYLINDER SUPPLY PILOT PRESSURIZED CLOSED OPEN

The poppets are actuated by cams on a camshaft (fig. 6-30). The camshaft is controlled by the movement of the handle. The valve may be operated by manually moving the handle, or, in some cases, the handle may be connected by mechanical linkage to a control handle which is located in a convenient place for the operator some distance from the valve. The camshaft may be rotated to any one of three positions (neutral and two working positions). In the neutral position the camshaft lobes are not contacting any of the poppets. This assures that the poppet springs will hold all four poppets firmly seated. With all poppets seated, there is no fluid flow through the valve. This also blocks the two cylinder ports; so when the valve is in neutral, the fluid in the actuating unit is trapped. Relief valves are installed in both working lines to prevent overpressurization caused by thermal expansion. NOTE: In some versions of this type of valve, the cam lobes are designed so that the two return/exhaust poppets are open when the valve is in the neutral position. This compensates for thermal expansion, because both working lines are open to the return/exhaust when the valve is in the neutral position. The poppets are arranged so that rotation of the camshaft will open the proper combination of poppets to direct the flow of fluid through the desired working line to an actuating unit. At the same time, fluid will be directed from the actuating unit through the opposite working line, through the valve, and back to the reservoir (hydraulic) or exhausted to the atmosphere (pneumatic). To stop rotation of the camshaft at an exact position, a stop pin is secured to the body and extends through a cutout section of the camshaft flange. This stop pin prevents overtravel by ensuring that the camshaft stops rotating at the point where the cam lobes have moved the poppets the greatest distance from their seats and where any further rotation would allow the poppets to start returning to their seats. O-rings are spaced at intervals along the length of the shaft to prevent external leakage around the ends of the shaft and internal leakage from one of the valve chambers to another. The camshaft has two lobes, or raised portions. The shape of these lobes is such that when the shaft is placed in the neutral position the lobes will not contact any of the poppets. When the handle is moved in either direction from neutral, the camshaft is rotated. This rotates Figure 6-30.—Cutaway view of poppet-type, four-way directional control valve. 6-21

the lobes, which unseat one pressure poppet and one return/exhaust poppet (fig. 6-31). The valve is now in the working position. Fluid under pressure, entering the pressure port, flows through the vertical fluid passages in both pressure poppets seats. Since only one pressure poppet, IN (2), is unseated by the cam lobe, the fluid flows past the open poppet to the inside of the poppet seat. From there it flows through the diagonal passages, out one cylinder port, C2, and to the actuating unit. Return fluid from the actuating unit enters the other cylinder port, C1. It then flows through the corresponding fluid passage, past the unseated return poppet, OUT (1), through the vertical fluid passages, and out the return/exhaust port. When the camshaft is rotated in the opposite direction to the neutral position, the two poppets seat and the flow stops. When the camshaft is further rotated in this direction until the stop pins hits, the opposite pressure and return poppets are unseated. This reverses the flow in the working lines, causing the actuating unit to move in the opposite direction. Rotary Spool Valve Four-way directional control valves of this type are frequently used as pilot valves to direct flow to and from other valves (fig. 6-32). Fluid is directed from one source of supply through the rotary valve to another directional control valve, where it positions the valve to direct flow from another source to one side of an actuating unit. Fluid from the other end of the main valve flows through a return line, through the rotary valve to the return or exhaust port. The principal parts of a rotary spool direc- tional control valve are shown in figure 6-22. Figure 6-31.—Working view of a poppet-type, four-way directional control valve. Figure 6-32.—Sliding spool valve controlled by a rotary spool valve. Figure 6-33 shows the operation of a rotary spool valve. Views A and C show the valve in a position to deliver fluid to another valve, while view B shows the valve in the neutral position, with all passages through the valve blocked. Rotary spool valves can be operated manually, electrically, or by fluid pressure. Sliding Spool Valve The sliding spool four-way directional control valve is similar in operation to the two-way valve previously described in this chapter. It is simple in its principle of operation and is the most durable and trouble-free of all four-way directional control valves. The valve described in the following para- graphs is a manually operated type. The same principle is used in many remotely controlled directional control valves. The valve (fig. 6-34) consists of a valve body containing four fluid ports—pressure (P), Figure 6-33.—Operation of a rotary spool, four-way directional control valve. 6-22 PRESSURE C1 OUT (1) OUT (2) IN (1) IN (2) C2 SELECTOR VALVE HANDLE FPf06031 RETURN ROTARY VALVE C1 P C2 SPOOL VALVE PUMP R FPf06032 A BC TO RESERVOIR TO RESERVOIR TO RESERVOIR TO VALVE TO VALVE FROM VALVE FROM PUMP FROM PUMP FROM PUMP FROM VALVE TO VALVE TO VALVE FPf06033

Figure 6-34.—Operation of a sliding spool, four-way directional control valve. 6-23

return/exhaust (R), and two cylinder ports (C/1 and C2). A hollow sleeve fits into the main bore of the body. There are O-rings placed at intervals around the outside diameter of the sleeve. These O-rings form a seal between the sleeve and the body, creating chambers around the sleeve. Each of the chambers is lined up with one of the fluid ports in the body. The drilled passage in the body accounts for a fifth chamber which results in having the two outboard chambers connected to the return/exhaust port. The sleeve has a pattern of holes drilled through it to allow fluid to flow from one port to another. A series of holes are drilled into the hollow center sleeve in each chamber. The sleeve is prevented from turning by a sleeve retainer bolt or pin which secures it to the valve body. The sliding spool fits into the hollow center sleeve. This spool is similar to the spool in the two-way valve, except that this spool has three pistons or lands. These lands are lapped or machine fitted to the inside of the sleeve. One end of the sliding spool is connected to a handle either directly or by mechanical linkage to a more desirable location. When the control handle is moved, it will position the spool within the sleeve. The lands of the spool then line up different combinations of fluid ports thus directing a flow of fluid through the valve. The detent spring is a clothespin-type spring, secured to the end of the body by a spring retaining bolt. The two legs of the spring extend down through slots in the sleeve and fit into the detents. The spool is gripped between the two legs of the spring. To move the spool, enough force must be applied to spread the two spring legs and allow them to snap back into the next detent, which would be for another position. Figure 6-34, view A, shows a manually operated sliding spool valve in the neutral position. The detent spring is in the center detent of the sliding spool. The center land is lined up with the pressure port (P) preventing fluid from flowing into the valve through this port. The return/exhaust port is also blocked, preventing flow through that port. With both the pressure and return ports blocked, fluid in the actuating lines is trapped. For this reason, a relief valve is usually installed in each actuating line when this type of valve is used. Figure 6-34, view B, shows the valve in the working position with the end of the sliding spool retracted. The detent spring is in the outboard detent, locking the sliding spool in this position. The lands have shifted inside the sleeve, and the ports are opened. Fluid under pressure enters the sleeve, passes through it by way of the drilled holes, and leaves through cylinder port C2. Return fluid, flowing from the actuator enters port C1, flows through the sleeve, and is directed out the return port back to the reservoir or exhausted to the atmosphere. Fluid cannot flow past the spool lands because of the lapped surfaces. Figure 6-34, view C, shows the valve in the opposite working position with the sliding spool extended. The detent spring is in the inboard detent. The center land of the sliding spool is now on the other side of the pressure port, and the fluid under pressure is directed through the sleeve and out port C1. Return fluid flowing in the other cylinder port is directed to the drilled passage in the body. It flows along this passage to the other end of the sleeve where it is directed out of the return/exhaust port. The directional control valves previously discussed are for use in closed-center fluid power systems. Figure 6-35 shows the operation of Figure 6-35.—Open center, sliding spool directional control valve. 6-24 C1 FPf06035 PC 2 TO RESERVOIR A C1 P C2 TO RESERVOIR B C1 P C2 TO RESERVOIR C

a representative open-center, sliding spool When the spool is moved to the right of the directional control valve. neutral position, view B, one working line (C1) is aligned to system pressure and the other When this type of valve is in the neutral working line (C2) is open through the hollow position (fig. 6-35, view A), fluid flows into the spool to the return port. View C shows the flow valve through the pressure port (P) through the of fluid through the valve with the spool moved hollow spool, and return to the reservoir. to the left of neutral. 6-25

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CHAPTER 7 SEALING DEVICES AND MATERIALS Recall from chapter 1 that Pascal’s theorem, from which the fundamental law for the science of hydraulics evolved, was proposed in the seventeenth century. One stipulation to make the law effective for practical applications was a piston that would “fit” the opening in the vessel “exactly.” However, it was not until the late eighteenth century that Joseph Brahmah invented an effective piston seal, the cup packing. This led to Brahmah's development of the hydraulic press. The packing was probably the most important invention in the development of hydraulics as a leading method of transmitting power. The development of machines to cut and shape closely fitted parts was also very important in the development of hydraulics. However, regardless of how precise the machining process is, some type of packing is usually required to make the piston, and many other parts of hydraulic components, “fit exactly.” This also applies to the components of pneumatic systems. Through years of research and experiments, many different materials and designs have been created in attempts to develop suitable packing devices. Suitable materials must be durable, must provide effective sealing, and must be compatible with the fluid used in the system. The packing materials are commonly referred to as seals or sealing devices. The seals used in fluid power systems and components are divided into two general classes-static seals and dynamic seals. The static seal is usually referred to as a gasket. The function of a gasket is to provide a material that can flow into the surface irregularities of mating areas that require sealing. To do this, the gasket material must be under pressure. This requires that the joint be tightly bolted or otherwise held together. The dynamic seal, commonly referred to as a packing, is used to provide a seal between two parts that move in relation to each other. These two classifications of seals—gaskets and packing—apply in most cases; however, deviations are found in some technical publi- cations. Certain types of seals (for example, the O-ring, which is discussed later) may be used either as a gasket or a packing. Many of the seals in fluid power systems prevent external leakage. These seals serve two purposes—to seal the fluid in the system and to keep foreign matter out of the system. Other seals simply prevent internal leakage within a system. NOTE: Although leakage of any kind results in a loss of efficiency, some leakage, especially internal leakage, is desired in hydraulic systems to provide lubrication of moving parts. This also applies to some pneumatic systems in which drops of oil are introduced into the flow of air in the system. The first part of this chapter deals primarily with the different types of materials used in the construction of seals. The next section is devoted to the different shapes and designs of seals and their application as gaskets and/or packings in fluid power systems. Also included in this chapter are sections concerning the functions of wipers and backup washers in fluid power systems and the selection, storage, and handling of sealing devices. SEAL MATERIALS As mentioned previously, many different materials have been used in the development of sealing devices. The material used for a particular application depends on several factors: fluid compatibility, resistance to heat, pressure, wear resistance, hardness, and type of motion. The selection of the correct packings and gaskets and their proper installation are important factors in maintaining an efficient fluid power system. The types of seals to be used in a particular piece of equipment is specified by the equipment manufacturer. 7-1

installations. A typical reservoir for use with ground and ship installations is shown in figure 9-1. This type of reservoir is made of hot rolled steel plates and has welded seams. The ends extend below the bottom of the reservoir and serve as supports. The bottom of the reservoir is convex, and a drain plug is incorporated at the lowest point. Nonpressurized reservoirs are also used in several transport-, patrol-, and utility-type aircraft. These aircraft are not designed for violent maneuvers and, in some cases, do not fly at high altitude. Those aircraft that have nonpressurized reservoirs installed and that fly at high altitudes have the reservoirs installed within a pressurized area. (High altitude in this situation means an altitude where atmospheric pressure is inadequate to maintain sufficient flow of fluid to the hydraulic pumps.) Most nonpressurized aircraft reservoirs are constructed in a cylindrical shape (fig. 9-2). The outer housing is manufactured from a strong corrosion-resistant metal. Filter elements are normally installed internally within the reservoir to clean returning system hydraulic fluid. Some of the older aircraft have a filter bypass valve installed to allow fluid to bypass the filter if the filter becomes clogged. Reservoirs that are filled by pouring fluid directly into them have a filler (finger) strainer assembly installed in the filler well to strain out impurities as the fluid enters the reservoir. Figure 9-2.—Nonpressurized aircraft reservoir. The quantity of fluid in the reservoir is indicated by either a glass tube, a directing gauge, or a float-type rod, which is visible through a transparent dome installed on the reservoir. PRESSURIZED RESERVOIRS A pressurized reservoir is required in hydraulic systems where atmospheric pressure is insufficient to maintain a net positive suction head (NPSH) to the pump. There are two common types of pressurized reservoirs—fluid-pressurized and air-pressurized. Fluid-Pressurized Reservoir Some aircraft hydraulic systems use fluid pressure for pressurizing the reservoir. The reservoir shown in figure 9-3 is of this type. This reservoir is divided into two chambers by a floating piston. The piston is forced downward in the reservoir by a compression spring within the pressurizing cylinder and by system pressure entering the pressurizing port of the cylinder. The pressurizing port is connected directly to the pressure line. When the system is pressurized, pressure enters the pressure port, thus pressurizing the reservoir. This pressurizes the pump suction line and the reservoir return line to the same pressure. The reservoir shown in figure 9-3 has five ports—pump suction, return, pressurizing, overboard drain, and bleed. Fluid is supplied to the pump through the pump suction port. Fluid returns to the reservoir from the system through the return port. Pressure from the pump enters the pressurizing cylinder in the top of the reservoir through the pressurizing port. The overboard drain port is used to drain the reservoir while performing maintenance, and the bleed port is used as an aid when servicing the reservoir. Air-Pressurized Reservoirs Air-pressurized reservoirs, such as the one shown in figure 9-4, are currently used in many high-performance naval aircraft. The reservoir is cylindrical in shape and has a piston installed internally to separate the air and fluid chambers. Air pressure is usually provided by engine bleed air. The piston rod end protrudes through the reservoir end cap and indicates the fluid quantity. The quantity indication may be seen by inspecting the distance the piston rod protrudes from the reservoir end cap. The reservoir is provided with 9-2 PUMP SUPPLY LINE EMERGENCY RESERVOIR VENT CHECK VALVE PUMP SUPPLY LINE VENT FILTER FPf09002

Figure 9-3.—Typical fluid-pressurized reservoir. threaded openings for connecting fittings and components. Figure 9-4 shows several components installed in lines leading to and from the reservoir; however, this may not be the case in actual installation. The air relief valve, bleeder valve, and soon, may reinstalled directly on the reservoir. Because the reservoir is pressurized, it can normally be installed at any altitude and still maintain a positive flow of fluid to the pump. Figure 9-4.—Air-pressurized reservoir. Some air-pressurized reservoirs also have direct contact of fluid to gas. These reservoirs are installed in large systems and may be cylindrical or rectangular in shape. They contain an oil level indicator, a pump inlet or suction line connection, a return line, a gas pressurization and venting connection, and a drain line connection or a drain plug. These reservoirs are pressurized by air from the ship’s service air system or nitrogen banks. These reservoirs are found on board aircraft carriers and submarines. ACCUMULATORS An accumulator is a pressure storage reservoir in which hydraulic fluid is stored under pressure from an external source. The storage of fluid under pressure serves several purposes in hydraulic systems. In some hydraulic systems it is necessary to maintain the system pressure within a specific pressure range for long periods of time. It is very difficult to maintain a closed system without some leakage, either external or internal. Even a small leak can cause a decrease in pressure. By using an accumulator, leakage can be compensated for 9-3 BLEED PORT (UP WHEN INSTALLED) PRESSURIZING PORT RETURN SUCTION BLEED VALVE COMPRESSION SPRING OVERBOARD DRAIN FITTING (DOWN WHEN INSTALLED) FPf09003 AIR BLENDER VALVE FLUID RETURN AIR SUPPLY FLUID SUPPLY AIR RELIEF VALVE INDICATOR FPf09004

Figure 9-5.–Cross-section view of a piston-type accumulator with a tailrod. 9-4 BLEED VENT PISTON ROD (TAIL ROD) CYLINDER HEAD SPLIT RING FOR RETAINING CYLINDER HEAD CYLINDER (BARREL) O-RING & BACKUP RINGS (NOT VISIBLE) PISTON SEAL & BACKUP RINGS PISTON PISTON SEAL & BACKUP RINGS QUAD-RING & BACKUP RINGS HEAD SEAL CYLINDER HEAD HP AIR CONNECTION AIR CHAMBER ACCUMULATOR SUPPORT TIE ROD OIL CHAMBER TAILROD SEAL & BACKUP RINGS HOLE FOR TIE ROD HYDRAULIC OIL CONNECTION FPf09005

and the system pressure can be maintained within an acceptable range for long periods of time. Accumulators also compensate for thermal expansion and contraction of the liquid due to variations in temperature. A liquid, flowing at a high velocity in a pipe will create a backward surge when stopped suddenly by the closing of a valve. This sudden stoppage causes instantaneous pressures two to three times the operating pressure of the system. These pressures, or shocks, produce objectional noise and vibrations which can cause considerable damage to piping, fittings, and components. The incorporation of an accumulator enables such shocks and surges to be absorbed or cushioned by the entrapped gas, thereby reducing their effects. The accumulator also dampens pressure surges caused by pulsating delivery from the pump. There are times when hydraulic systems require large volumes of liquid for short periods of time. This is due to either the operation of large cylinders or the necessity of operating two or more circuits simultaneously. It is not economical to install a pump of such large capacity in the system for only intermittent usage, particularly if there is sufficient time during the working cycle for an accumulator to store up enough liquid to aid the pump during these peak demands. The energy stored in accumulators maybe also used to actuate hydraulically operated units if normal hydraulic system failure occurs. Four types of accumulators used in Navy hydraulic systems are as follows: 1. Piston type 2. Bag or bladder type 3. Direct-contact gas-to-fluid type 4. Diaphragm type PISTON-TYPE ACCUMULATORS Piston-type accumulators consist of a cylindrical body called a barrel, closures on each end called heads, and an internal piston. The piston may be fitted with a tailrod, which extends through one end of the cylinder (fig. 9-5), or it may not have a tailrod at all (fig. 9-6). In the latter case, it is referred to as a floating piston. Hydraulic fluid is pumped into one end of the cylinder and the piston is forced toward the opposite end of the cylinder against a captive Figure 9-6.—Floating piston-type accumulator. 9-5 END CAP PACKING AND BACKUP RING BARREL ASSEMBLY AIR PORT PISTON LUBRICATION PASSAGE END CAP HYDRAULIC FLUID PORT FPf09006

charge of air or an inert gas such as nitrogen. Sometimes the amount of air charge is limited to the volume within the accumulator; other installations may use separate air flasks which are piped to the air side of the accumulator. Piston accumulators may be mounted in any position. The gas portion of the accumulator may be located on either side of the piston. For example, in submarine hydraulic systems with tailrod pistons, the gas is usually on the bottom and the fluid on top; in surface ships with floating pistons, the gas is usually on the top. The orientation of the accumulator and the type of accumulator are based upon such criteria as available space, maintenance accessibility, size, need for external monitoring of the piston’s location (tailrod indication), contamination tolerance, seal life, and safety. The purpose of the piston seals is to keep the fluid and the gas separate. Usually, tailrod accumulators use two piston seals, one for the air side and one for the oil side, with the space between them vented to the atmosphere through a hole drilled the length of the tailrod. When the piston seals fail in this type of accumulator, air or oil leakage is apparent. However, seal failure in floating piston or nonvented tailrod accumulators will not be as obvious. Therefore, more frequent attention to venting or draining the air side is necessary. An indication of worn and leaking seals can be detected by the presence of significant amounts of oil in the air side. BLADDER-TYPE ACCUMULATORS Bladder- or bag-type accumulators consist of a shell or case with a flexible bladder inside the shell. See figure 9-7. The bladder is larger in diameter at the top (near the air valve) and gradually tapers to a smaller diameter at the bottom. The synthetic rubber is thinner at the top of the bladder than at the bottom. The operation of the accumulator is based on Barlow’s formula for hoop stress, which states: “The stress in a circle is directly proportional to its diameter and wall thickness.” This means that for a certain thickness, a large diameter circle will stretch faster than a small diameter circle; or for a certain diameter, a thin wall hoop will stretch faster than a thick wall hoop. Thus, the bladder will stretch around the top at its largest diameter and thinnest wall thickness, and then will gradually stretch downward and push itself outward against the walls of the shell. As a result, the bladder is capable of squeezing out all the liquid from. Figure 9-7.—Bladder-type accumulator. the accumulator. Consequently, the bladder accumulator has a very high volumetric efficiency. In other words, this type of accumulator is capable of supplying a large percentage of the stored fluid to do work. The bladder is precharged with air or inert gas to a specified pressure. Fluid is then forced into the area around the bladder, further compressing the gas in the bladder. This type of accumulator has the advantage that as long as the bladder is intact there is no exposure of fluid to the gas charge and therefore less danger of an explosion. DIRECT-CONTACT GAS-TO-FLUID ACCUMULATORS Direct-contact gas-to-fluid accumulators generally are used in very large installations where it would be very expensive to require a piston- or bladder-type accumulator. This type of accumulator consists of a fully enclosed cylinder, mounted in a vertical position, containing a liquid 9-6 AIR LIQUID FPf09007

port on the bottom and a pneumatic charging port at the top (fig. 9-8). This type of accumulator is used in some airplane elevator hydraulic systems where several thousand gallons of fluid are needed to supplement the output of the hydraulic pumps for raising the elevator platform. The direct contact between the air or gas and the hydraulic fluid tends to entrain excessive amounts of gas in the fluid. For this reason, direct contact accumulators are generally not used for pressures over 1200 psi. The use of this type of accumulator with flammable fluid is dangerous because there is a possibility of explosion if any oxygen is present in the gas, and pressure surges generate excessive heat. For this reason, safety fluids are used in this type of installation. DIAPHRAGM ACCUMULATORS The diaphragm-type accumulator is con- structed in two halves which are either screwed or bolted together. A synthetic rubber diaphragm is installed between both halves, making two chambers. Two threaded openings exist in the assembled component. The opening at the top, as shown in figure 9-9, contains a screen disc which prevents the diaphragm from extruding through the threaded opening when system pressure is depleted, thus rupturing the dia- phragm. On some designs the screen is replaced by a button-type protector fastened to the center Figure 9-8.—Direct-contact gas-to-fluid accumulator. Figure 9-9.—Diaphragm accumulator. of the diaphragm. An air valve for pressurizing the accumulator is located in the gas chamber end of the sphere, and the liquid port to the hydraulic system is located on the opposite end of the sphere. This accumulator operates in a manner similar to that of the bladder-type accumulator. FILTRATION You have learned that maintaining hydraulic fluids within allowable limits is crucial to the care and protection of hydraulic equipment. While every effort must be made to prevent contaminants from entering the system, con- taminants which do find their way into the system must be removed. Filtration devices are installed at key points in fluid power systems to remove the contaminants that enter the system along with those that are generated during normal operations. Filtration devices for hydraulic systems differ somewhat from those of pneumatic systems. Therefore, they will be discussed separately. The filtering devices used in hydraulic systems are commonly referred to as strainers and filters. Since they share a common function, the terms strainer and filter are often used interchangeably. As a general rule, devices used to remove large particles of foreign matter from hydraulic fluids are referred to as strainers, while those used to remove the smallest particles are referred to as filters. 9-7 HIGH-PRESSURE AIR LINE FOR PRECHARGING FLUID LEVEL FPf09008 TO HYDRAULIC SYSTEM SCREEN DISC DIAPHRAGM AIR CHAMBER AIR VALVE CAP FLIUD CHAMBER PRESSURE LINE FPf09009

STRAINERS Strainers are used primarily to catch only very large particles and will be found in applications where this type of protection is required. Most hydraulic systems have a strainer in the reservoir at the inlet to the suction line of the pump. A strainer is used in lieu of a filter to reduce its chance of being clogged and starving the pump. However, since this strainer is located in the reservoir, its maintenance is frequently neglected. When heavy dirt and sludge accumulate on the suction strainer, the pump soon begins to cavitate. Pump failure follows quickly. FILTERS The most common device installed in hydraulic systems to prevent foreign particles and contamination from remaining in the system are referred to as filters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides they are needed to safeguard the system against impurities. Filters are classified as full flow and proportional or partial flow. In the full-flow type of filter, all the fluid that enters the unit passes through the filtering element, while in the proportional-flow type, only a portion of the fluid passes through the element. Full-Flow Filter The full-flow filter provides a positive filtering action; however, it offers resistance to flow, particularly when the element becomes dirty. Hydraulic fluid enters the filter through the inlet port in the body and flows around the filter element inside the filter bowl. Filtering takes place as the fluid passes through the filtering element and into the hollow core, leaving the dirt and impurities on the outside of the filter element. The filtered fluid then flows from the hollow core through the outlet port and into the system (fig. 9-10). Some full-flow filters are equipped with a contamination indicator (fig. 9-11). These indicators, also known as differential pressure indicators, are available in three types—gauge indicators, mechanical pop-up indicators, and electrical with mechanical pop-up indicators. As contaminating particles collect on the filter element, the differential pressure across the element increases. In some installations using Figure 9-10.—Full-flow hydraulic filter. gauges as indicators, the differential pressure must be obtained by subtracting the readings of two gauges located somewhere along the filter inlet and outlet piping. For pop-up indicators, when the increase in pressure reaches a specific value, an indicator (usually in the filter head) pops out, signifying that the filter must be cleaned or replaced. A low-temperature lockout feature is installed in most pop-up types of contamination indicators to eliminate the possibility of false indications due to cold weather because the pressure differential may be much higher with a cold fluid due to increased viscosity. Filter elements used in filters that have a contamination indicator are not normally removed or replaced until the indicator is actuated. This decreases the possibility of system contamination from outside sources due to unnecessary handling. The use of the nonbypassing type of filter eliminates the possibility of contaminated fluid bypassing the filter element and contaminating the entire system. This type of filter will minimize the necessity for flushing the entire system and lessen the possibility of failure of pumps and other components in the system. A bypass relief valve is installed in some filters. The bypass relief valve allows the fluid to bypass the filter element and pass directly through the outlet port in the event that the filter element becomes clogged. These filters may or may not be equipped with the contamination indicator. Figure 9-11 shows a full-flow bypass-type 9-8 OUT BODY FILTER ELEMENT FILTER BOWL IN FPf09010

Figure 9-11.—Full-flow bypass-type hydraulic filter (with contamination indicator). 9-9 RED DIFFERENTIAL PRESSURE INDICATOR BUTTON (RAISES 3/16 INCH WHEN DIFFERENTIAL PRESSURE EXCEEDS 70 ( 10) lb/in INDICATING A CLOGGED FILTER ELEMENT). -+ 2 MAGNETIC ACTUATOR (TYPICAL) INLET OUTLET FILTER BYPASS RELIEF VALVE OPENS AT A DIFFERENTIAL PRESSURE OF 100 ( 10) lb/in 2+- FILTER ELEMENTFILTER BOWL VIEW A NORMAL FLOW SPRING FILTER HEAD POPPET SPRING SHUTOFF DIAPHRAGM SPRING BOWL O-RING VIEW B BYPASS FLOW (CLOGGED FILTER ELEMENT) BYPASSED FLUID UNFILTERED FLUID FILTERED FLUID FPf09011

hydraulic filter with a contamination indicator. Figure 9-12 shows a full-flow bypass-type hydraulic filter without a contamination indicator. A filter bypass indicator provides a positive indication, when activated, that fluid is bypassing the filter element by flowing through the bypass relief valve. This indicator should not be confused with the pop-up differential pressure indicator previously discussed which simply monitors the pressure across the element. With the bypass indicator, a similar pop-up button is often used to signal that maintenance is needed. However, the bypass indicators further signal that, as a result of the high differential pressures across the element, an internal bypass relief valve has lifted and some of the fluid is bypassing the element. Identification of the type of installed indicator can be obtained from filter manifold drawings or related equipment manuals. Both a fluid bypass indicator and a differential pressure indicator or gauge may be installed on the same filter assembly. As with differential pressure indicators, bypass relief indicators can be activated by pressure surges, such as may develop during cold starts or rapid system pressurization. On some relief indicators, the pop-up button, or whatever signal device is used, will return to a normal position when the surge passes and pressure is reduced. Other relief indicators may continue to indicate a bypass condition until they are manually reset. Figure 9-12.—Full-flow bypass-type hydraulic filter. Before corrective action is taken based on indicator readings, the bypass condition should be verified at normal operating temperature and flow conditions by attempting to reset the indicator. Proportional-Flow Filter This type of filter operates on the venturi principle. (See glossary.) As the fluid passes through the venturi throat a drop in pressure is created at the narrowest point. See figure 9-13. A portion of the fluid flowing toward and away from the throat of the venturi flows through the passages into the body of the filter. A fluid passage connects the hollow core of the filter with the throat of the venturi. Thus, the low-pressure area at the throat of the venturi causes the fluid under pressure in the body of the filter to flow through the filter element, through the hollow core, into the low-pressure area, and then return to the system. Although only a portion of the fluid is filtered during each cycle, constant recirculation through the system will eventually cause all the fluid to pass through the filter element. Figure 9-13.Proportional-flow filter. 9-10 IN OUT BYPASS RELIEF VALVE (50 PSI) BODY FILTER ELEMENT FILTER BOWL FPf09012 VENTURI THROAT BAFFLE BODY CARTRIDGE SEDIMENT FPf09013

Filter Rating Filters are rated in several ways—absolute, mean, and nominal. The absolute filtration rating is the diameter in microns of the largest spherical particle that will pass through the filter under a certain test condition. This rating is an indication of the largest opening in the filter element. The mean filtration rating is the measurement of the average size of the openings in the filter element. The nominal filtration rating is usually interpreted to mean the size of the smallest particles of which 90 percent will be trapped in the filter at each pass through the filter. Filter Elements Filter elements generally may be divided into two classes—surface and depth. Surface filters are made of closely woven fabric or treated paper with a uniform pore size. Fluid flows through the pores of the filter material and contaminants are stopped on the filter’s surface. This type of element is designed to prevent the passage of a high percentage of solids of a specific size. Depth filters, on the other hand, are composed of layers of fabric or fibers which provide many tortuous paths for the fluid to flow through. The pores or passages must be larger than the rated size of the filter if particles are to be retained in the depth of the media rather than on the surface. Consequently, there is a statistical probability that a rather large particle may pass through a depth-type filter. Filter elements may be of the 5-micron, woven mesh, micronic, porous metal, or magnetic type. The micronic and 5-micron elements have noncleanable filter media and are disposed of when they are removed. Porous metal, woven mesh, and magnetic filter elements are usually designed to be cleaned and reused. 5-MICRON NONCLEANABLE FILTER ELEMENTS.— The most common 5-micron filter medium is composed of organic and inorganic fibers integrally bonded by epoxy resin and faced with a metallic mesh upstream and downstream for protection and added mechanical strength. Filters of this type are not to be cleaned under any circumstances and will be marked Disposable or Noncleanable. Another 5-micron filter medium uses layers of very fine stainless-steel fibers drawn into a random but controlled matrix. Filter elements Figure 9-14.—Cross-section of a stainless steel hydraulic filter element. of this material may be either cleanable or noncleanable, depending upon their construction. WOVEN WIRE-MESH FILTER ELE- MENTS.— Filters of this type are made of stainless steel and are generally rated as 15 or 25 micron (absolute). Figure 9-14 shows a magnified cross section of a woven wire-mesh filter element. This type of filter is reusable. MICRONIC HYDRAULIC FILTER ELE- MENT.— The term micronic is derived from the word micron. It could be used to describe any filter element; however, through usage, this term has become associated with a specific filter with a filtering element made of a specially treated cellulose paper (fig. 9-15). The filter shown in figure 9-10 is a typical micronic hydraulic filter. This filter is designed to remove 99 percent of all particles 10 to 20 microns in diameter or larger. Figure 9-15.—Micronic filter element. 9-11 0.0055 IN. FPf09014 FPf09015

The replaceable element is made of specially treated convolutions (wrinkles) to increase its dirt-holding capacity. The element is noncleanable and should be replaced with a new filter element during maintenance inspections. MAGNETIC FILTERS.— Some hydraulic systems have magnetic filters installed at strategic points. Filters of this type are designed primarily to trap any ferrous particles that may be in the system. PNEUMATIC GASES Clean, dry gas is required for the efficient operation of pneumatic systems. Due to the normal conditions of the atmosphere, free air seldom satisfies these requirements adequately. The atmosphere contains both dust and impurities in various amounts and a substantial amount of moisture in vapor form. Solids, such as dust, rust, or pipe scale in pneumatic systems, may lead to excessive wear and failure of components and, in some cases, may prevent the pneumatic devices from operating. Moisture is also very harmful to the system. It washes lubrication from moving parts, thereby aiding corrosion and causing excessive wear of components. Moisture will also settle in low spots in the system and freeze during cold weather, causing a stoppage of the system or ruptured lines. An ideal filter would remove all dirt and moisture from a pneumatic system without causing a pressure drop in the process. Obviously, such a condition can only be approached; it cannot be attained. Removal of Solids The removal of solids from the gas of pneumatic systems is generally done by screening (filtering), centrifugal force, or a combination of the two. In some cases, the removal of moisture is done in conjunction with the removal of solids. Some types of air filters are similar in design and operation to the hydraulic filters discussed earlier. Some materials used in the construction of elements for air filters are woven screen wire, steel wool, fiber glass, and felt fabrics. Elements made of these materials are often used in the unit that filters the air as it enters the compressor. Porous metal and ceramic elements are commonly used in filters that are installed in the compressed air supply lines. These filters also use a controlled air path to provide some filtration. Internal design causes the air to flow in a circular path within the bowl (fig. 9-16). Heavy particles and water droplets are thrown out of the airstream and drop to the bottom of the bowl. The air then flows through the filter element, which filters out most of the smaller particles. This type of filter is designed with a drain valve at the bottom of the bowl. When the valve is opened with air pressure in the system, the accumulation of solids and water will be blown out of the bowl. An air filter that uses moving mechanical devices as an element is illustrated in figure 9-17. As compressed air passes through the filter the force revolves a number of multi-blade rotors at high speed. Moisture and dirt are caught on the blades of the rotors. The whirling blades hurl the impurities by centrifugal force to the outer rims of the rotors and to the inner walls of the filter housing. Here, contaminating matter is out of the airstream and falls to the bottom of the bowl where it must be drained at periodic intervals. Removal of Moisture The removal of moisture from compressed air is important for a compressed air system. If air at atmospheric pressure, even at a very low relative humidity, is compressed to 3000 or 4500 psi, it becomes saturated with water vapor. Some moisture is removed by the intercoolers and aftercoolers (see glossary). Also, air flasks, receivers, and banks are provided with low point drains to allow periodic draining of any collected moisture. However, many uses of air require air with an even smaller moisture content than can be obtained through these methods. Moisture in Figure 9-16.—Air filter. 9-12 OUTLET INLET FPf09016

Figure 9-17.—Air filter using rotating blades as element. air lines can create problems which are potentially hazardous, such as the freezing of valves and controls. This can occur, for example, if very high pressure air is throttled to a very low pressure at a high flow rate. The venturi effect of the throttled air produces very low temperatures which will cause any moisture in the air to freeze into ice. This makes the valve (especially an automatic valve) either very difficult or impossible to operate. Also, droplets of water can cause serious water hammer in an air system which has high pressure and a high flow rate and can cause corrosion, rust, and dilution of lubricants within the system. For these reasons, air driers (dehydrator, air purifier, and desiccator are all terms used by different manufacturers to identify these components) are used to dry the compressed air. Some water removal devices are similar in design and operation to the filters shown in figures 9-16 and 9-17. Two basic types of air dehydrators are the refrigerated-type and the desiccant-type. REFRIGERATED-TYPE DEHYDRATORS.– In refrigerated-type dehydrators, compressed air is passed over a set of refrigerated cooling coils. Oil and moisture vapors condense from the air and can be collected and removed via a low point drain. DESICCANT-TYPE DEHYDRATORS.– A desiccant is a chemical substance with a high capacity to absorb water or moisture. It also has the capacity to give off that moisture so that the desiccant can be reused. Some compressed air system dehydrators use a pair of desiccant towers (flasks full of desiccant). One is kept in service dehydrating the compressed air, while the other one is being reactivated. A desiccant tower is normally reactivated by passing dry, heated air through it in the direction opposite the normal dehydration airflow. Another type of chemical drier is shown in figure 9-18. This unit consists of the housing, a cartridge containing a chemical agent, a filter (sintered bronze), and a spring. Various types of absorbent chemicals are used by the different manufacturers in the construction of the cartridges. To ensure proper filtering, the air must pass through the drier in the proper direction. The correct direction of flow is indicated by an arrow and the word FLOW printed on the side of the cartridge. Figure 9-18.–Chemical drier. 9-13 OUT IN DRAINFPf09017 CARTRIDGE HOUSING SPRING OUTLET FILTER FILTERDESICCANT FLOW FLOW DRIER CARTRIDGE FILTERCARTRIDGE CARTRIDGE AND HOUSING CARTRIDGEFPf09018 INLET

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CHAPTER 10 ACTUATORS One of the outstanding features of fluid power systems is that force, generated by the power supply, controlled and directed by suitable valving, and transported by lines, can be con- verted with ease to almost any kind of mechanical motion desired at the very place it is needed. Either linear (straight line) or rotary motion can be obtained by using a suitable actuating device. An actuator is a device that converts fluid power into mechanical force and motion. Cylinders, motors, and turbines are the most common types of actuating devices used in fluid power systems. This chapter describes various types of actuating cylinders and their applications, different types of fluid motors, and turbines used in fluid power systems. CYLINDERS An actuating cylinder is a device that converts fluid power to linear, or straight line, force and motion. Since linear motion is a back-and-forth motion along a straight line, this type of actuator is sometimes referred to as a reciprocating, or linear, motor. The cylinder consists of a ram or piston operating within a cylindrical bore. Actuat- ing cylinders may be installed so that the cylinder is anchored to a stationary structure and the ram or piston is attached to the mechanism to be operated, or the piston or ram may be anchored to the stationary structure and the cylinder attached to the mechanism to be operated. Actuating cylinders for pneumatic and hydraulic systems are similar in design and operation. Some of the variations of ram- and piston-type actuating cylinders are described in the following paragraphs. RAM-TYPE CYLINDERS The terms ram and piston are often used interchangeably. However, a ram-type cylinder is usually considered one in which the cross-sectional area of the piston rod is more than one-half the cross-sectional area of the movable element. In most actuating cylinders of this type, the rod and the movable element have equal areas. This type of movable element is frequently referred to as a plunger. The ram-type actuator is used primarily to push rather than to pull. Some applications require simply a flat surface on the external part of the ram for pushing or lifting the unit to be operated. Other applications require some mechanical means of attachment, such as a clevis or eyebolt. The design of ram-type cylinders varies in many other respects to satisfy the requirements of different applications. Single-Acting Ram The single-acting ram (fig. 10-1) applies force in only one direction. The fluid that is directed into the cylinder displaces the ram and forces it outward, lifting the object placed on it. Since there Figure 10-1.—Single-acting ram-type actuating cylinder. 10-1 FPf10001 PACKED GLAND FLUID SUPPLY RAM GLAND NUT CYLINDER HOUSING

is no provision for retracting the ram by fluid power, when fluid pressure is released, either the weight of the object or some mechanical means, such as a spring, forces the ram back into the cylinder. This forces the fluid back to the reservoir. The single-acting ram-type actuating cylinder is often used in the hydraulic jack. The elevators used to move aircraft to and from the flight deck and hangar deck on aircraft carriers also use cylinders of this type. In these elevators, the cylinders are installed horizontally and operate the elevator through a series of cables and sheaves. Fluid pressure forces the ram outward and lifts the elevator. When fluid pressure is released from the ram, the weight of the elevator forces the ram back into the cylinder. This, in turn, forces the fluid back into the reservoir. Double-Acting Ram A double-acting ram-type cylinder is illustrated in figure 10-2. In this cylinder, both strokes of the ram are produced by pressurized fluid. There are two fluid ports, one at or near each end of the cylinder. Fluid under pressure is directed to the closed end of the cylinder to extend the ram and apply force. To retract the ram and reduce the force, fluid is directed to the opposite end of the cylinder. A four-way directional control valve is normally used to control the double-acting ram. When the valve is positioned to extend the ram, pressurized fluid enters port A (fig. 10-2), acts on Figure 10-2.—Double-acting ram-type actuating cylinder. the bottom surface of the ram, and forces the ram outward. Fluid above the ram lip is free to flow out of port B, through the control valve, and to the return line in hydraulic systems or to the atmosphere in pneumatic systems. Normally, the pressure of the fluid is the same for either stroke of the ram. Recall from chapter 2 that force is equal to pressure times area (F= PA). Notice the difference of the areas upon which the pressure acts in figure 10-2. The pressure acts against the large surface area on the bottom of the ram during the extension stroke, during which time the ram applies force. Since the ram does not require a large force during the retraction stroke, pressure acting on the small area on the top surface of the ram lip provides the necessary force to retract the ram. Telescoping Rams Figure 10-3 shows a telescoping ram-type actuating cylinder. A series of rams is nested in the telescoping assembly. With the exception of the smallest ram, each ram is hollow and serves as the cylinder housing for the next smaller ram. The ram assembly is contained in the main cylinder assembly, which also provides the fluid ports. Although the assembly requires a small space with all the rams retracted, the telescoping action of the assembly provides a relatively long stroke when the rams are extended. An excellent example of the application of this type of cylinder is in the dump truck. It is used to lift the forward end of the truck bed and dump the load. During the lifting operation, the greatest force is required for the initial lifting of the load. Figure 10-3.—Telescoping ram-type actuating cylinder. 10-2 PACKED GLAND RAM GLAND NUT CYLINDER HOUSING PORT B PORT A RAM LIP FLUID SUPPLY FOR EXTENDING FLUID SUPPLY FOR RETRACTION FPf10002 FPf10003 RAM 1 RAM 2 RAM 3 PORT A RAISE LOWER PORT B

As the load is lifted and begins to dump, the required force becomes less and less until the load is completely dumped. During the raise cycle, pressurized fluid enters the cylinder through port A (fig. 10-3) and acts on the bottom surface of all three rams. Ram 1 has a larger surface area and, therefore, provides the greater force for the initial load, As ram 1 reaches the end of its stroke and the required force is decreased, ram 2 moves, providing the smaller force needed to continue raising the load. When ram 2 completes its stroke, a still smaller force is required. Ram 3 then moves outward to finish raising and dumping the load. Some telescoping ram-type cylinders are of the single-acting type. Like the single-acting ram discussed previously, these telescoping ram-type cylinders are retracted by gravity or mechanical force. Some hydraulic jacks are equipped with telescoping rams. Such jacks are used to lift vehicles with low clearances to the required height. Other types of telescoping cylinders, like the one illustrated in figure 10-3, are of the double- acting type. In this type, fluid pressure is used for both the extension and retraction strokes. A four- way directional control valve is commonly used to control the operation of the double-acting type. Note the small passages in the walls of rams 1 and 2. They provide a path for fluid to flow to and from the chambers above the lips of rams 2 and 3. During the extension stroke, return fluid flows through these passages and out of the cylinder through port B. It then flows through the directional control valve to the return line or reservoir. To retract the rams, fluid under pressure is directed into the cylinder through port B and acts against the top surface areas of all three ram lips. This forces the rams to the retracted position. The displaced fluid from the opposite side of the rams flows out of the cylinder through port A, through the directional control valve to the return line or reservoir. Dual Rams A dual ram assembly consists of a single ram with a cylinder at either end (fig. 10-4). Fluid can be directed to either cylinder, forcing the ram to move in the opposite direction. The ram is connected through mechanical linkage to the unit to be operated. A four-way directional control valve is commonly used to operate the dual ram. When the control valve is positioned to direct fluid under pressure to one of the cylinders (let’s say the left one), the ram is forced to the right. This Figure 10-4.-Dual ram actuating assembly. action displaces the fluid in the opposite cylinder. The displaced fluid flows back through the directional control valve to the return line or reservoir in hydraulic systems or to the atmosphere in pneumatic systems. Dual ram actuating assemblies are used in steering systems of most ships. In some systems, one assembly is used to actuate the rudder in either direction; while in other systems, two assemblies are used for the same purpose. PISTON-TYPE CYLINDERS An actuating cylinder in which the cross- sectional area of the piston is less than one-half the cross-sectional area of the movable element is referred to as a piston-type cylinder. This type of cylinder is normally used for applications that require both push and pull functions. The piston- type cylinder is the most common type used in fluid power systems. The essential parts of a piston-type cylinder are a cylindrical barrel, a piston and rod, end caps, and suitable seals. The end caps are attached to the ends of the barrel. These end caps usually contain the fluid ports. The end cap on the rod end contains a hole for the piston rod to pass through. Suitable seals are used between the hole and the piston rod to keep fluid from leaking out and to keep dirt and other contaminants from entering the barrel. The opposite end cap of most cylinders is provided with a fitting for securing the actuating cylinder to some structure. This end cap is referred to as the anchor end cap. The piston rod may extend through either or both ends of the cylinder. The extended end of the rod is normally threaded so that some type of mechanical connector, such as an eyebolt or a clevis, and a locknut can be attached. This threaded connection of the rod and mechanical connector provides for adjustment between the rod and the unit to be actuated. After the correct 10-3 FPf10004 SHIP'S RUDDER RAM CYLINDER CYLINDER FLUID SUPPLY PIPE FLUID SUPPLY PIPE

adjustment is made, the locknut is tightened against the connector to prevent the connector from turning. The other end of the connector is attached, either directly or through additional mechanical linkage, to the unit to be actuated. In order to satisfy the many requirements of fluid power systems, piston-type cylinders are available in various designs. Single-Acting Cylinder The single-acting piston-type cylinder is similar in design and operation to the single-acting ram-type cylinder. The single-acting piston-type cylinder uses fluid pressure to provide the force in one direction, and spring tension, gravity, compressed air, or nitrogen is used to provide the force in the opposite direction. Figure 10-5 shows a single-acting, spring-loaded, piston-type actuating cylinder. In this cylinder the spring is located on the rod side of the piston. In some spring-loaded cylinders the spring is located on the blank side, and the fluid port is on the rod side of the cylinder. A three-way directional control valve is normally used to control the operation of the single-acting piston-type cylinder. To extend the piston rod, fluid under pressure is directed through the port into the cylinder (fig. 10-5). This pressure acts on the surface area of the blank side of the piston and forces the piston to the right. This action moves the rod to the right, through the end of the cylinder, thus moving the actuated unit in one direction. During this action, the spring is compressed between the rod side of the piston and the end of the cylinder. The length of the stroke depends upon the physical limits within the cylinder and the required movement of the actuated unit. To retract the piston rod, the directional control valve is moved to the opposite working position, which releases the pressure in the Figure 10-5.—Single-acting, spring-loaded, piston-type actuating cylinder. cylinder. The spring tension forces the piston to the left, retracting the piston rod and moving the actuated unit in the opposite direction. The fluid is free to flow from the cylinder through the port, back through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems. The end of the cylinder opposite the fluid port is vented to the atmosphere. This prevents air from being trapped in this area. Any trapped air would compress during the extension stroke, creating excess pressure on the rod side of the piston. This would cause sluggish movement of the piston and could eventually cause a complete lock, preventing the fluid pressure from moving the piston. The spring-loaded cylinder is used in arresting gear systems on some models of carrier aircraft. To raise (retract) the arresting hook, fluid pressure is directed through the arresting hook control valve to the rod side of the cylinder. This force moves the piston, which, through the rod and mechanical linkage, retracts the arresting hook. The arresting hook extends when fluid pressure is released from the rod side of the cylinder, allowing the spring to expand. Leakage between the cylinder wall and piston is prevented by adequate seals. The piston in figure 10-5 contains V-ring seals. Double-Acting Cylinder Most piston-type actuating cylinders are double-acting, which means that fluid under pressure can be applied to either side of the piston to apply force and provide movement. One design of the double-acting cylinder is shown in figure 10-6. This cylinder contains one piston and piston rod assembly. The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder, alternate as inlet and outlet ports, depending on the direction of Figure 10-6.-Doub1e-acting piston-type actuating cylinder. 10-4 FPf10005 FLUID PORT PISTON RETURN SPRING PISTON ROD AIR VENT SEAL FPf10006 PISTON FLUID PORTS O-RING SEAL CLEVIS

flow from the directional control valve. This actuator (fig. 10-6) is referred to as an unbalanced actuating cylinder because there is a difference in the effective working areas on the two sides of the piston. Therefore, this type of cylinder is normally installed so that the blank side of the piston carries the greater load; that is, the cylinder carries the greater load during the piston rod extension stroke. A four-way directional control valve is normally used to control the operation of this type of cylinder. The valve can be positioned to direct fluid under pressure to either end of the cylinder and allow the displaced fluid to flow from the opposite end of the cylinder through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems. There are applications where it is necessary to move two mechanisms at the same time. In this case, double-acting piston-type actuating cylinders of different designs are required. See figures 10-7 and 10-8. Figure 10-7 shows a three-port, double-acting piston-type actuating cylinder. This actuator contains two pistons and piston rod assemblies. Fluid is directed through port A by a four-way directional control valve and moves the pistons outward, thus moving the mechanisms attached to the pistons’ rods. The fluid on the rod side of each piston is forced out of the cylinder through ports B and C, which are connected by a common line to the directional control valve. The displaced fluid then flows through the control valve to the return line or to the atmosphere. When fluid under pressure is directed into the cylinder through ports B and C, the two pistons move inward, also moving the mechanisms attached to them. Fluid between the two pistons is free to flow from the cylinder through port A and through the control valve to the return line or to the atmosphere. The actuating cylinder shown in figure 10-8 is a double-acting balanced type. The piston rod extends through the piston and out through both ends of the cylinder. One or both ends of the Figure 10-7.—Three-port, double-acting actuating cylinder. Figure 10-8.-Balanced, double-acting piston-type actuating cylinder. piston rod may be attached to a mechanism to be operated. In either case, the cylinder provides equal areas on each side of the piston. Therefore, the same amount of fluid and force is used to move the piston a certain distance in either direction. Tandem Cylinders A tandem actuating cylinder consists of two or more cylinders arranged one behind the other but designed as a single unit (fig. 10-9). This type of actuating cylinder is used in applications that require two or more independent systems; for example, power-operated flight control systems in naval aircraft. The flow of fluid to and from the two chambers of the tandem actuating cylinder is provided from two independent hydraulic systems and is controlled by two sliding spool directional control valves. In some applications, the control valves and the actuating cylinder are two separate units. In some units, the pistons (lands) of the two sliding spools are machined on one common shaft. In other applications, the valves and the actuator are directly connected in one compact unit. Although the two control valves are hydraulically independent, they are interconnected mechanically. In other units, the two sliding spools are connected through mechanical linkages with a synchronizing rod. In either case, the movement of the two sliding spools is synchronized, thus equalizing the Figure 10-9.—Tandem actuating cylinder. 10-5 FPf10007 PORT B TO CONTROL VALVE PORT A PORT C PISTON FPf10008 SHAFT SHAFT O-RING SEAL FLUID PORTS FPf10009 AB CD

flow of fluid to and from the two chambers of the actuating cylinder. Since the two control valves operate independently of each other as far as hydraulic pressure is concerned, failure of either hydraulic system does not render the actuator inoperative. Failure of one system does reduce the output force by one-half; however, this force is sufficient to permit operation of the actuator. RACK-AND-PINION PISTON-TYPE ROTARY ACTUATORS The rack-and-pinion-type actuators, also referred to as limited rotation cylinders, of the single or multiple, bidirectional piston are used for turning, positioning, steering, opening and closing, swinging, or any other mechanical function involving restricted rotation. Figure 10-10 shows a typical rack-and-pinion double- piston actuator. The actuator consists of a body and two reciprocating pistons with an integral rack for rotating the shaft mounted in roller or journal bearings. The shaft and bearings are located in a central position and are enclosed with a bearing cap. The pistons, one on each side of the rack, are enclosed in cylinders machined or sleeved into the body. The body is enclosed with end caps and static seals to prevent external leakage of pressurized fluid. Only a few of the many applications of actuating cylinders were discussed in the preceding paragraphs. Figure 10-11 shows additional types of force and motion applications. In addition to its versatility, the cylinder-type actuator is probably the most trouble-free component of fluid power systems. However, it is very important that the cylinder, mechanical linkage, and actuating unit are correctly aligned. Any misalignment will cause excessive wear of the piston, piston rod, and seals. Also, proper adjustment between the piston rod and the actuating unit must be maintained. Figure 10-10.—Rack-and-pinion double-piston rotary actuator. 10-6 FPf10010 B B SECTION BB PISTONS AND RACKS CYLINDER PINION GEAR SEALS DYNAMIC PORT INLET BACKUP RINGS SEALS STATIC PORT OUTLET CONDUIT INTERNAL

Figure 10-11.—Applications of actuating cylinders. 10-7 CYLINDER CAN BE USED WITH A THIRD CLASS LEVER. CYLINDER CAN BE USED WITH A TRAMMEL PLATE. DOUBLE SPROCKET WHEEL MAKES THE ROTATION MORE NEARLY CONTINUOUS. CYLINDER CAN BE LINKED UP DIRECTLY TO THE LOAD. POINT OF APPLICATION OF FORCE FOLLOWS THE DIRECTION OF THRUST . MOTION IS TRANSMITTED TO A DISTANT POINT IN THE PLANE OF MOTION. SIMULTANEOUS THRUSTS IN TWO DIFFERENT DIRECTIONS ARE OBTAINED. TWO PISTONS WITH FIXED STROKES POSITION LOAD IN ANY OF FOUR STATIONS. MOTION OF MOVABLE RACK IS TWICE THAT OF PISTON. CYLINDER CAN BE USED WITH A FIRST CLASS LEVER. A TOGGLE CAN BE ACTUATED BY THE CYLINDER. GEAR SECTOR MOVES RACK PERPENDICULAR TO STROKE OF PISTON. FPf10011

MOTORS controlled by either a four-way directional control valve or a variable-displacement pump. A fluid power motor is a device that converts fluid power energy to rotary motion and force. The function of a motor is opposite that of a pump. However, the design and operation of fluid power motors are very similar to pumps. Therefore, a thorough knowledge of the pumps described in chapter 4 will help you understand the operation of fluid power motors. Motors have many uses in fluid power systems. In hydraulic power drives, pumps and motors are combined with suitable lines and valves to form hydraulic transmissions. The pump, commonly referred to as the A-end, is driven by some outside source, such as an electric motor. The pump delivers fluid to the motor. The motor, referred to as the B-end, is actuated by this flow, and through mechanical linkage conveys rotary motion and force to the work. This type of power drive is used to operate (train and elevate) many of the Navy’s guns and rocket launchers. Hydraulic motors are commonly used to operate the wing flaps, radomes, and radar equipment in aircraft. Air motors are used to drive pneumatic tools. Air motors are also used in missiles to convert the kinetic energy of compressed gas into electrical power, or to drive the pump of a hydraulic system. Fluid motors may be either fixed or variable displacement. Fixed-displacement motors provide constant torque and variable speed. The speed is varied by controlling the amount of input flow. Variable-displacement motors are constructed so that the working relationship of the internal parts can be varied to change displacement. The majority of the motors used in fluid power systems are the fixed-displacement type. Although most fluid power motors are capable of providing rotary motion in either direction, some applications require rotation in only one direction. In these applications, one port of the motor is connnected to the system pressure line and the other port to the return line or exhausted to the atmosphere. The flow of fluid to the motor is controlled by a flow control valve, a two-way directional control valve, or by starting and stopping the power supply. The speed of the motor may be controlled by varying the rate of fluid flow to it. In most fluid power systems, the motor is required to provide actuation power in either direction. In these applications the ports are referred to as working ports, alternating as inlet and outlet ports. The flow to the motor is usually Fluid motors are usually classified according to the type of internal element, which is directly actuated by the flow. The most common types of elements are the gear, the vane, and the piston, AU three of these types are adaptable for hydraulic systems, while only the vane type is used in pneumatic systems. GEAR-TYPE MOTORS The spur, helical, and herringbone design gears are used in gear-type motors. The motors use external-type gears, as discussed in chapter 4. The operation of a gear-type motor is shown in figure 10-12. Both gears are driven gears; however, only one is connected to the output shaft. As fluid under pressure enters chamber A, it takes the path of least resistance and flows around the inside surface of the housing, forcing the gears to rotate as indicated. The flow continues through the outlet port to the return. This rotary motion of the gears is transmitted through the attached shaft to the work unit. The motor shown in figure 10-12 is operating in one direction; however, the gear-type motor is capable of providing rotary motion in either direction. To reverse the direction of rotation, the ports may be alternated as inlet and outlet. When fluid is directed through the outlet port (fig. 10-12) into chamber B, the gears rotate in the opposite direction. Figure 10-12.—Gear-type motor. 10-8 FPf10012 OUTLET INLET B A 12

VANE-TYPE MOTORS A typical vane-type air motor is shown in figure 10-13. This particular motor provides rotation in only one direction. The rotating element is a slotted rotor which is mounted on a drive shaft. Each slot of the rotor is fitted with a freely sliding rectangular vane. The rotor and vanes are enclosed in the housing, the inner surface of which is offset from the drive shaft axis. When the rotor is in motion, the vanes tend to slide outward due to centrifugal force. The distance the vanes slide is limited by the shape of the rotor housing. This motor operates on the principle of differential areas. When compressed air is directed into the inlet port, its pressure is exerted equally in all directions. Since area A (fig. 10-13) is greater than area B, the rotor will turn counterclockwise. Each vane, in turn, assumes the No. 1 and No. 2 positions and the rotor turns continuously. The potential energy of the compressed air is thus converted into kinetic energy in the form of rotary motion and force. The air at reduced pressure is exhausted to the atmosphere. The shaft of the motor is connected to the unit to be actuated. Many vane-type motors are capable of providing rotation in either direction. A motor of this design is shown in figure 10-14. This motor operates on the same principle as the vane motor shown in figure 10-13. The two ports may be alternately used as inlet and outlet, thus providing rotation in either direction. Note the springs in the slots of the rotor. Their purpose is to hold the vanes against the housing during the initial Figure 10-13.—Vane-type air motor. Figure 10-14.—Vane-type motor. starting of the motor, since centrifugal force does not exist until the rotor begins to rotate. PISTON-TYPE MOTORS Piston-type motors are the most commonly used in hydraulic systems. They are basically the same as hydraulic pumps except they are used to convert hydraulic energy into mechanical (rotary) energy. The most commonly used hydraulic motor is the fixed-displacement piston type. Some equipment uses a variable-displacement piston motor where very wide speed ranges are desired. Although some piston-type motors are controlled by directional control valves, they are often used in combination with variable- displacement pumps. This pump-motor combina- tion is used to provide a transfer of power between a driving element and a driven element. Some applications for which hydraulic transmissions may be used are speed reducers, variable speed drives, constant speed or constant torque drives, and torque converters. Some advantages of hydraulic transmission of power over mechanical transmission of power are as follows: 1. 2. 3. 4. Quick, easy speed adjustment over a wide range while the power source is operating at a constant (most efficient) speed. Rapid, smooth acceleration or deceleration. Control over maximum torque and power. Cushioning effect to reduce shock loads. Smoother reversal of motion. 10-9 FPf10013 INLET 1 2 AREA "A" AREA "B" VANE CHAMBER WALL HOUSING EXHAUST ROTOR FPf10014 SPRINGS SHAFT INLET ROTOR OUTLET CASING VANES

Radial-Piston Motor continues as long as fluid under pressure enters the cylinders. The radial-piston motor operates in reverse of the radial-piston pump. In the radial-piston pump, as the cylinder block rotates, the pistons press against the rotor and are forced in and out of the cylinders, thereby receiving fluid and pushing it out into the system. In the radial motor, fluid is forced into the cylinders and drives the pistons outward. The pistons pushing against the rotor cause the cylinder block to rotate. The operation of a radial-piston motor is shown in figure 10-15. This motor is shown with three pistons for simplicity. Normally it contains seven or nine pistons. When liquid is forced into the cylinder bore containing piston 1, the piston moves outward since the liquid cannot be compressed. This causes the cylinder to rotate in a clockwise direction. As the force acting on piston 1 causes the cylinder block to rotate, piston 2 starts to rotate and approach the position of piston 3. (Note that the distance between the cylinder block and the reaction ring of the rotor gets progressively shorter on the top and right half of the rotor.) As piston 2 rotates, it is forced inward and, in turn, forces the fluid out of the cylinder. Since there is little or no pressure on this side of the pintle valve, the piston is easily moved in by its contact with the reaction ring of the rotor. The fluid is easily forced out of the cylinder and back to the reservoir or to the inlet side of the pump. As the piston moves past the midpoint, or past the shortest distance between the cylinder block and the rotor, it enters the pressure side of the pintle valve and fluid is forced into the cylinder. Piston 3 then becomes the pushing piston and in turn rotates the cylinder block. This action Figure 10-15.—Operation of a radial-piston motor. The direction of rotation of the motor (fig. 10-15) is changed by reversing the flow of fluid to it. Admitting fluid under pressure on the top side of the pintle valve forces piston 3 out of the cylinder block. This causes the cylinder to rotate in the counterclockwise direction. Axial-Piston Motor The variable-stroke axial-piston pump is often used as a part of variable speed gear, such as electrohydraulic anchor windlasses, cranes, winches, and the power transmitting unit in electrohydraulic steering engines. In those cases, the tilting box is arranged so that it maybe tilted in either direction. Thus it maybe used to transmit bidirectional power hydraulically to pistons or rams, or it may be used to drive a hydraulic motor. In the latter use, the pump is the A-end of the variable speed gear and the hydraulic motor is the B-end. The B-end of the hydraulic unit of the hydraulic speed gear is exactly the same as the A-end of the variable-stroke pump mentioned previously. However, it generally does not have a variable-stroke feature. The tilting box is installed at a permanently fixed angle. Thus, the B-end becomes a fixed-stroke axial-piston motor. Figure 10-16 illustrates an axial-piston hydraulic speed gear with the A-end and B-end as a single unit. It is used in turrets for train and elevation driving units. For electrohydraulic winches and cranes, the A-end and B-end are in separate housings connected by hydraulic piping. Hydraulic fluid introduced under pressure to a cylinder (B-end) tries to push the piston out of the cylinder. In being pushed out, the piston, through its piston rod, will seek the point of greatest distance between the top of the cylinder and the socket ring. The resultant pressure of the piston against the socket ring will cause the cylinder barrel and the socket ring to rotate. This action occurs during the half revolution while the piston is passing the intake port of the motor, which is connected to the pressure port of the pump. After the piston of the motor has taken all the hydraulic fluid it can from the pump, the piston passes the valve plate land and starts to discharge oil through the outlet ports of the motor 10-10

Figure 10-16.—Exploded view of a axial-piston hydraulic speed gear. to the suction pistons of the pump. The pump is constantly putting pressure on one side of the motor and receiving hydraulic fluid from the other side. The fluid is merely circulated from pump to motor and back again. Both of the axial-piston motors described in this section may be operated in either direction. The direction of rotation is controlled by the direction of fluid flow to the valve plate. The direction of flow may be instantly reversed without damage to the motor. TURBINES Turbines are used in pneumatic systems to convert kinetic energy of gases to mechanical energy. Turbines are used to drive electric generators, to convert mechanical energy into electrical energy, and to drive pumps to supply fluid flow in hydraulic systems. The basic parts of a turbine are the rotor, which has blades projecting radially from its periphery; and nozzles, through which the gas is expanded and directed. The conversion of kinetic energy to mechanical energy occurs on the blades. The basic distinction between types of turbines is the manner in which the gas causes the turbine rotor to move. When the rotor is moved by a direct push or “impulse” from the gas impinging upon the blades, the turbine is said to be an impulse turbine. When the rotor is moved by force of reaction, the turbine is said to be a reaction turbine. Although the distinction between impulse turbines and reaction turbines is a useful one, it should not be considered as an absolute distinction in real turbines. An impulse turbine uses both the impulse of the gas jet and, to a lesser extent, the reactive force that results when the curved blades cause the gas to change direction. A reaction turbine is moved primarily by reactive force, but some motion of the rotor is caused by the impact of the gas against the blades. IMPULSE TURBINE The impulse turbine consists essentially of a rotor mounted on a shaft that is free to rotate in a set of bearings. The outer rim of the rotor carries a set of curved blades, and the whole assembly is enclosed in an airtight case. Nozzles direct the 10-11

rapidly moving fluid against the blades and turn the rotor (fig. 10-17). REACTION TURBINE The reaction turbine, as the name implies, is turned by reactive force rather than by a direct push or impulse. In reaction turbines, there are no nozzles as such. Instead, the blades that project radially from the periphery of the rotor are formed and mounted so that the spaces between the blades have, in cross section, the shape of nozzles. Since these blades are mounted on the revolving rotor, they are called moving blades. Fixed or stationary blades of the same shape as the moving blades (fig. 10-18) are fastened to the stator (casing) in which the rotor revolves. The fixed blades guide the gas into the moving blade system and, since they are also shaped and mounted to provide nozzle-shaped spaces between the blades, the freed blades also act as nozzles. A reaction turbine is moved by three main forces: (1) the reactive force produced on the moving blades as the gas increases in velocity as it expands through the nozzle-shaped spaces between the blades; (2) the reactive force produced on the moving blades when the gas changes direction; and (3) the push or impulse of the gas impinging upon the blades. Thus, as previously noted, a reaction turbine is moved primarily by reactive force but also to some extent by direct impulse. Impulse and reaction blades can be combined to form an impulse-reaction turbine. This turbine combines the rotational forces of the previously described turbines; that is, it derives its rotation from both the impulse of the gas striking the turbine blades and the reactive force of the gas changing direction. Figure 10-17 .—Impulse turbine. Figure 10-18.—Reaction turbine blading. 10-12 FPf10018 FIXED BLADES ENTERING GAS MOVING BLADES

CHAPTER 11 PNEUMATICS The word pneumatics is a derivative of the Greek word pneuma, which means air, wind, or breath. It can be defined as that branch of engineering science that pertains to gaseous pressure and flow. As used in this manual, pneumatics is the portion of fluid power in which compressed air, or other gas, is used to transmit and control power to actuating mechanisms. This chapter discusses the origin of pneu- matics. It discusses the characteristics of gases and compares them with those of liquids. It also explains factors which affect the properties of gases, identifies and explains the gas laws, and identifies gases commonly used in pneumatics and their pressure ranges. It also discusses hazards of pneumatic gases, methods of controlling contami- nation, and safety precautions associated with compressed gases. DEVELOPMENT OF PNEUMATICS There is no record of man’s first uses of air to do work. Probably the earliest uses were to separate chaff from grain and to move ships. One of the first pneumatic devices was the blow gun used by primitive man. In the latter part of the eighteenth century, heated air was used to carry the first balloon aloft. The heated air, being lighter than the surrounding air, caused the balloon to rise. Every age of man has witnessed the develop- ment of devices which used air to do work. However, man used air to do work long before he understood it. Many of the principles of hydraulics apply to pneumatics. For example, Pascal’s law applies to gases as well as liquids. Also, like hydraulics, the development of pneumatics depended on closely fitted parts and the development of gaskets and packings. Since the invention of the air com- pressor, pneumatics has become a very reliable way to transmit power. Probably one of the most common uses of pneumatic power is in the operation of pneumatic tools. However, you should understand that pneumatics is also of great importance in large and complex systems such as the controls of vital propulsion and weapon systems. CHARACTERISTICS OF GASES Recall from chapter 1 that gas is one of the three states of matter. It has characteristics similar to those of liquids in that it has no definite shape but conforms to the shape of its container and readily transmits pressure. Gases differ from liquids in that they have no definite volume. That is, regardless of the size or shape of the containing vessel, a gas will completely fill it. Gases are highly compressible, while liquids are only slightly so. Also, gases are lighter than equal volumes of liquids, making gases less dense than liquids. DENSITY Early experiments were conducted concerning the behavior of air and similar gases. These experiments were conducted by scientists such as Boyle and Charles (discussed later in this chapter). The results of their experiments indicated that the gases’ behavior follows the law known as the ideal-gas law. It states as follows: For a given weight of any gas, the product of the absolute pressure and the volume occupied, divided by the absolute temperature, is constant. In equation form, it is expressed as follows: Equation 11-1 For 1 pound of gas, Equation 11-2 11-1

The specific volume (v) is expressed in cubic feet per pound. For any weight of a gas this equation maybe modified as follows: W = weight of the gas in pounds, V = volume of W pounds of the gas in cubic feet. The volume of 1 pound would then be V/W. If we substitute this for v in equation 11-3, it then becomes Solving equation 11-4 for pressure, In chapter 2 we defined density as the mass per unit volume. In equation 11-5, represents density. (Notice that this is the reverse of the specific volume.) We can now say that pressure is equal to the density of the gas times the gas constant times the absolute temperature of the gas. (The gas constant varies for different gases.) From this equation we can show how density varies with changes in pressure and temperature. Decreasing the volume, with the weight of the gas and the temperature held constant, causes the pressure to increase. NOTE: During the compression of the gas, the temperature will actually increase; however, the explanation is beyond the scope of this text. a decrease in volume with the weight held constant will cause density to increase. TEMPERATURE As indicated previously, temperature is a dominant factor affecting the physical properties of gases. It is of particular concern in calculating changes in the states of gases. Three temperature scales are used extensively in gas calculations. They are the Celsius (C), the Fahrenheit (F), and the Kelvin (K) scales. The Celsius (or centigrade) scale is constructed by identifying the freezing and boiling points of water, under standard conditions, as fixed points of 0° and 100°, respectively, with 100 equal divisions between. The Fahrenheit scale identifies 32° as the freezing point of water and 212° as the boiling point, and has 180 equal divisions between. The Kelvin scale has its zero point equal to –273°C, or –460°F. Absolute zero, one of the fundamental constants of physics, is commonly used in the study of gases. It is usually expressed in terms of the Celsius scale. If the heat energy of a gas sample could be progressively reduced, some temperature should be reached at which the motion of the molecules would cease entirely. If accurately determined, this temperature could then be taken as a natural reference, or as a true absolute zero value. Experiments with hydrogen indicated that if a gas were cooled to –273.16°C (–273° for most calculations), all molecular motion would cease and no additional heat could be extracted. Since this is the coldest temperature to which an ideal gas can be cooled, it is considered to be absolute zero. Absolute zero may be expressed as 0°K, –273°C, or –459.69°F (–460°F for most calculations). When you work with temperatures, always be sure which system of measurement is being used and how to convert from one to another. The conversion formulas are shown in figure 11-1. For purposes of calculations, the Rankine (R) scale illustrated in figure 11-1 is commonly used to 11-2

Figure 11-1.-Comparison of Kelvin, Celsius, Fahrenheit, and Rankine temperature. convert Fahrenheit to absolute. For Fahrenheit readings above zero, 460° is added. Thus, 72°F equals 460° plus 72°, or 532° absolute (532°R). If the Fahrenheit reading is below zero, it is subtracted from 460°. Thus, -40°F equals 460° minus 40°, or 420° absolute (420°R). The Kelvin and Celsius scales are used internationally in scientific measurements; there- fore, some technical manuals may use these scales in directions and operating instructions. The Fahrenheit scale is commonly used in the United States; therefore, it is used in most areas of this manual. PRESSURE We defined pressure in chapter 2 as force per unit area. Remember, liquids exert pressure on all surfaces with which they come in contact. Gases, because of their ability to completely fill containers, exert pressure on all sides of a container. In practice, we maybe interested in either of two pressure readings. We may desire either the gauge pressure or the absolute pressure. Absolute pressure is measured from absolute zero pressure rather than from normal or atmospheric pressure (approximately 14.7 psi). Gauge pressure is used on all ordinary gauges, and indicates pressure in excess of atmospheric pressure. Therefore, absolute pressure is equal to atmospheric pressure plus gauge pressure. For example, 100 psi gauge pressure (psig) equals 100 psi plus 14.7 psi or 114.7 psi absolute pressure (psia). Whenever gas laws are applied, absolute pressures Gases are required. COMPRESSIBILITY AND EXPANSION OF GASES can be readily compressed and are assumed to be perfectly elastic. This combination of properties gives a gas the ability to yield to a 11-3 FPf11001 373 100 273 492 -273 -459.4 212 0 0 0 32 672 TEMPERATURE CONVERSION KELVIN TO CELSIUS AND FAHRENHEIT FAHRENHEIT TO KELVIN AND CELSIUS 9 5 9 5 9 5 F= ( K-2 7 3 )+3 2 = K - 459.4 = C+3 2 K = C +273 = (F - 32) + 273 = F + 255.23 5 9 5 9 o CELSIUS TO KELVIN AND FAHRENHEIT 5 9 C=K-2 7 3 = (F - 32) o o KELVIN CELSIUS FARENHEIT RANKINE WATER BOILS ICE MELTS

force and return promptly to its original condition when the force is removed. These are the properties of air that is used in pneumatic tires, tennis balls and other deformable objects whose shapes are maintained by compressed air. KINETIC THEORY OF GASES In an attempt to explain the compressibility of gases, Bernoulli proposed the hypothesis that is accepted as the kinetic theory of gases. According to this theory, the pressure exerted by a gas on the walls of a closed container is caused by continual bombardment of the walls by molecules of the gas. Consider the container shown in figure 11-2 as containing a gas. At any given time, some molecules are moving in one direction, some are traveling in other directions; some are traveling fast, some slow, and some may even be in a state of rest. The average effect of the molecules bombarding each container wall corresponds to the pressure of the gas. As more gas is pumped into the container, more molecules are available to bombard the walls; thus the pressure in the container increases. The gas pressure in a container can also be increased by increasing the speed with which the molecules hit the walls. If the temperature of the gas is raised, the molecules move faster causing an increase in pressure. This can be shown by considering the automobile tire. When you take a long drive on a hot day, the pressure in the tires increases and a tire which appeared to be somewhat “soft” in cool morning temperature may appear normal at a higher midday tempera- ture. BOYLE’S LAW When the automobile tire is initially inflated, air which normally occupies a specific volume is compressed into a smaller volume inside the tire. This increases the pressure on the inside of the tire. Charles Boyle, an English scientist, was among the first to experiment with the pressure-volume relationship of gas. During an experiment when he compressed a volume of air he found that the volume decreased as the pressure increased, and by doubling the force exerted on the air he could decrease the volume of the air by half. See figure 11-3. Recall from the example of the automobile tire that changes in temperature of a gas also change the pressure and volume. Therefore, the experiment must be performed at a constant temperature. The relationship between pressure and volume is known as Boyle’s law. It states: When the temperature of a gas is kept constant, the volume of an enclosed gas varies inversely with its pressure. In equation form, this relationship may be expressed as either or Equation 11-6 where V 1 and P 1 are the original volume and pressure, and V 2 and P 2 are the final volume and pressure (P 1 and P2 are absolute pressures). Figure 11-3.-Gas compressed to half its original volume by Figure 11-2.—Molecular bombardment creating pressure. 11-4 a doubled force. FPf11002 FPf11003 V V 2 GAS F 2F

Example of Boyle’s law: 4 cubic feet of nitrogen are under a pressure of 100 psi (gauge). The nitrogen is allowed to expand to a volume of 6 cubic feet. What is the new gauge pressure? Remember to convert gauge pressure to absolute pressure by adding 14.7. Using equation 11-6, V 1P1 = V 2P2, where V 1 is 4 ft 3 , V 2 is 6 ft, and P 1 is 100 psig: CHARLES’S LAW Boyle’s law assumes conditions of constant temperature. In actual situations this is rarely the case. Temperature changes continually and affects the volume of a given mass of gas. Jacques Charles, a French physicist, provided much of the foundation for the modern kinetic theory of gases. Through experiments, he found that all gases expand and contract proportionally to the change in the absolute temperature, providing the pressure remains constant. The relationship between volume and temperature is known as Charles’s law. It states: The volume of a gas is proportional to its absolute temperature, if constant pressure is maintained. In equation form, this relationship may be expressed as Equation 11-7 where V 1 and V 2 are the original and final volumes, and T 1 and T2 are the original and final absolute temperatures. Since an increase in the temperature of a gas causes it to expand if the pressure is kept constant, it is reasonable to expect that if a given sample is heated within a closed container and its volume remains constant, the pressure of the gas will increase. Experiments have proven this to be true. In equation form, this becomes P 1 T 2 = P 2 T 1 Equation 11-8 or This equation states that for a constant volume, the absolute pressure of a gas varies directly with the absolute temperature. Example: A cylinder of gas under a pressure of 1800 psig at 70°F is left out in the sun in the tropics and heats up to a temperature of 130°F. What is the new pressure within the cylinder? (Remember that both pressure and temperature must be converted to absolute pressure and absolute temperature.) Converting absolute pressure to gauge pressure: 11-5

GENERAL GAS LAW We have learned that Boyle’s law pertains to situations in which the temperature remains constant (fig. 11-4), and that Charles’s law pertains to situations in which pressure remains constant (fig. 11-4). It is usually not possible to control pressure or temperature in tanks or bottles of gas subject to the weather and shipboard demands. Boyle’s and Charles’s laws are com- bined to form the general gas law. This law states: The product of the initial pressure, initial volume, and new temperature (absolute scale) of an enclosed gas is equal to the product of the new pressure, new volume, and initial temperature. It is a mathematical statement which allows many gas problems to be solved by using the principles of Boyle’s law and/or Charles’s law. The equation is expressed as or (P and T represent absolute pressure and absolute temperature, respectively.) You can see by examining figure 11-4 that the three equations are special cases of the general equation. Thus, if the temperature remains constant, T1 equals T2 and both can be eliminated from the general formula, which then reduces to the form shown in part A. When the volume remains constant, V 1 equals V2, thereby reducing Figure 11-4.—The general gas law. the general equation to the form given in part B. Similarly, P 1 is equated to P 2 for constant pressure, and the equation then takes the form given in part C. The general gas law applies with exactness only to “ideal” gases in which the molecules are assumed to be perfectly elastic. However, it describes the behavior of actual gases with sufficient accuracy for most practical purposes. Two examples of the general equation follow: 1. Two cubic feet of a gas at 75 psig and 80°F are compressed to a volume of 1 cubic foot and then heated to a temperature of 300°F. What is the new gauge pressure? Using equation 11-9, P 1V1T2 = P 2V2T1, where V1 is 2 ft 3 , P 1 is 75 psig, T 1 is 80°F, V 2 is 1 ft 3 and T 2 is 300°F: Solution: Substituting: Converting absolute pressure to gauge pressure: 2. Four cubic feet of a gas at 75 psig and 80°F are compressed to 237.8 psig and heated to a temperature of 300°F. What is the volume of the gas resulting from these changes? Using equation 11-9, P 1V1T2 = P2V2T1, where V 1 is 4 ft 3 , P 2 is 11-6 FPf11004 VOLUME CONSTANT PRESSURE CONSTANT TEMPERATURE CONSTANT PV =P V11 2 2 P T V T P T V T 1 1 1 1 2 2 2 2 = = ABC

75 psig, T 1 is 800, P 1 is 237.8 psig, and T 2 is 300°F: Solution: Substituting: PNEUMATIC GASES In chapter 1, you learned that many factors are considered in determining whether to use hydraulics or pneumatics as a power source in a fluid power system. Once it is determined that pneumatics will be used as the source of power, some of the same factors are considered in selecting the pneumatic gas. QUALITIES The ideal fluid medium for a pneumatic system is a readily available gas that is nonpoisonous (nontoxic), chemically stable, free from any acids that cause corrosion of system components, and nonflammable. It also will not support combustion of other elements. Gases that have these desired qualities may not have the required lubricating power. Therefore, lubrication of the components of some pneumatic systems must be arranged by other means. For example, some air compressors are provided with a lubricating system, some components are lubricated upon installation or, in some cases, lubrication is introduced into the air supply line. Two gases meeting these qualities and most commonly used in pneumatic systems are com- pressed air and nitrogen. COMPRESSED AIR Compressed air is a mixture of all gases contained in the atmosphere. In this manual, compressed air is referred to as a gas when it is used as a fluid medium. The unlimited supply of air and the ease of compression make compressed air the most widely used fluid for pneumatic systems. Although moisture and solid particles must be removed from the air, it does not require the extensive distillation or separation process required in the production of other gases. Compressed air has most of the desired properties and characteristics of a gas for pneumatic systems. It is nonpoisonous and nonflammable but does contain oxygen, which supports combustion. One of the most undesirable qualities of compressed air as a fluid medium for pneumatic systems is moisture content. The atmosphere contains varying amounts of moisture in vapor form. Changes in the temperature of compressed air will cause condensation of moisture in the pneumatic system. This condensed moisture can be very harmful to the system, as it increases corrosion, dilutes lubricants, and may freeze in lines and components during cold weather. Moisture separators and air driers (dehydrators) are installed in the compressed air lines to minimize or eliminate moisture in systems where moisture would deteriorate system performance. The supply of compressed air at the required volume and pressure is provided by an air compressor. (For information on air compressors, refer to Naval Ships’ Technical Manual, chapter 551.) In most systems the compressor is part of the system with distribution lines leading from the compressor to the devices to be operated. In these systems a receiver is installed in-line between the compressor and the device to be operated to help eliminate pulsations in the compressor discharge line, to act as a storage tank during intervals when the demand for air exceeds the compressor’s capacity, and to enable the compressor to shut down during periods of light load. Other systems receive their supply from cylinders which must be filled at a centrally located air compressor and then connected to the system. Compressed air systems are categorized by their operating pressures as follows: high-pressure (HP) air, medium-pressure (MP) air, and low- pressure (LP) air. High-Pressure Air Systems HP air systems provide compressed air at a nominal operating pressure of 3000 psi or 5000 psi and are installed whenever pressure in excess 11-7

of 1000 psi is required. HP compressed air plants support functions which require high pressures and high flow rates of compressed air by the addition of HP storage flasks to the system. An example of such a system is one that provides air for starting diesel and gas turbine engines. Reduction in pressure, if required, is done by using specially designed pressure-reducing stations. Medium-Pressure Air MP air systems provide compressed air at a nominal operating pressure of 151 psi to 1000 psi. These pressures are provided either by an MP air compressor or by the HP air system supplying air through an air bank and pressure-reducing stations. Low-Pressure Air LP air systems provide compressed air at a nominal operating pressure of 150 psi and below. The LP air system is supplied with LP air by LP air compressors or by the HP air system supplying air through an air bank and pressure-reducing stations. LP air is the most extensive and varied air system used in the Navy, In addition to being used for various pneumatic applications, LP and HP compressed air are used in the production of nitrogen. NITROGEN For all practical purposes, nitrogen is considered to be an inert gas. It is nonflammable, does not form explosive mixtures with air or oxygen, and does not cause rust or decay. Due to these qualities, its use is preferred over compressed air in many pneumatic systems, especially aircraft and missile systems, and wherever an inert gas blanket is required. Nitrogen is obtained by the fractional distillation of air. Oxygen/nitrogen-producing plants expand compressed air until its temperature decreases to –196°C (–320°F), the boiling point of nitrogen at atmospheric pressure. The liquid nitrogen is then directed to a storage tank. A liquid nitrogen pump pumps the low-pressure liquid nitrogen from the storage tank and discharges it as a high-pressure (5000 psi) liquid to the vaporizer where it is converted to a gas at 5000 psi. Oxygen/nitrogen-producing plants are located at many naval installations and on submarine tenders and aircraft carriers. CONTAMINATION CONTROL As in hydraulic systems, fluid contamination is also a leading cause of malfunctions in pneumatic systems. In addition to the solid particles of foreign matter which find a way to enter the system, there is also the problem of moisture. Most systems are equipped with one or more devices to remove this contamination. These include filters, water separators, air dehydrators, and chemical driers, which are discussed in chapter 9 of this manual. In addition, most systems contain drain valves at critical low points in the system. These valves are opened periodically to allow the escaping gas to purge a large percentage of the contaminants, both solids and moisture, from the system. In some systems these valves are opened and closed automatically, while in others they must be operated manually. Complete purging is done by removing lines from various components throughout the system and then attempting to pressurize the system, causing a high rate of airflow through the system. The airflow will cause the foreign matter to be dislodged and blown from the system. NOTE: If an excessive amount of foreign matter, particularly oil, is blown from any one system, the lines and components should be removed and cleaned or replaced. In addition to monitoring the devices installed to remove contamination, it is your responsibility as a maintenance person or supervisor to control the contamination. You can do this by using the following maintenance practices: 1. Keep all tools and the work area in a clean, dirt-free condition. 2. Cap or plug all lines and fittings immediately after disconnecting them. 3. Replace all packing and gaskets during assembly procedures. 4. Connect all parts with care to avoid stripping metal slivers from threaded areas. Install and torque all fittings and lines according to applicable technical instructions. 5. Complete preventive maintenance as specified by MRCs. Also, you must take care to ensure that the proper cylinders are connected to systems being supplied from cylinders. Cylinders for compressed air are painted black. Cylinders containing oil-pumped air have 11-8

two green stripes painted around the top of the cylinder, while cylinders containing water-pumped air have one green stripe. Oil-pumped air indicates that the air or nitrogen is compressed by an oil-lubricated compressor. Air or nitrogen com- pressed by a water-lubricated (or nonlubricated) compressor is referred to as water pumped. Oil-pumped nitrogen can be very dangerous in certain situations. For example, nitrogen is commonly used to purge oxygen systems. Oxygen will not burn, but it supports and accelerates combustion and will cause oil to burn easily and with great intensity. Therefore, oil-pumped nitrogen must never be used to purge oxygen systems. When the small amount of oil remaining in the nitrogen comes in contact with the oxygen, an explosion may result. In all situations, use only the gas specified by the manufacturer or recommended by the Navy. Nitrogen cylinders are painted gray. One black stripe identifies cylinders for oil-pumped nitrogen, and two black stripes identify cylinders for water-pumped nitrogen. In addition to these color codes, the exact identi- fication of the contents is printed in two locations diametrically opposite one another along the longitudinal axis of the cylinder. For compressed air and nitrogen cylinders, the lettering is white. POTENTIAL HAZARDS All compressed gases are hazardous. Com- pressed air and nitrogen are neither poisonous nor flammable, but should not be handled carelessly. Some pneumatic systems operate at pressures exceeding 3000 psi. Lines and fittings have exploded, injuring personnel and property. Literally thousands of careless workers have blown dust or harmful particles into their eyes by the careless handling of compressed air outlets. Nitrogen gas will not support life, and when it is released in a confined space, it will cause asphyxia (the loss of consciousness as a result of too little oxygen and too much carbon dioxide in the blood). Although compressed air and nitrogen seem so safe in comparison with other gases, do not let overconfidence lead to personal injury. SAFETY PRECAUTIONS To minimize personal injury and equipment damage when using compressed gases, observe all practical operating safety precautions, including the following: 1. Do not use compressed air to clean parts of your body or clothing, or to perform general space cleanup in lieu of vacuuming or sweeping. 2. Never attempt to stop or repair a leak while the leaking portion is still under pressure. Always isolate, repressurize and danger tag out the portion of the system to be repaired. For pressures of 1000 psi or greater, double valve protection is required to prevent injury if one of the valves should fail. 3. Avoid the application of heat to the air piping system or components, and avoid striking a sharp or heavy blow on any pressurized part of the piping system. 4. Avoid rapid operation of manual valves. The heat of compression caused by a sudden high- -pressure flow into an empty line or vessel can cause an explosion if oil is present. Valves should be slowly cracked open until airflow is noted and should be kept in this position until pressures on both sides of the valve have equalized. The rate of pressure rise should be kept under 200 psi per second, if possible. Valves may then be opened fully. 5. Do not discharge large quantities of nitrogen into closed compartments unless adequate ventilation is provided. 6. Do not subject compressed gas cylinders to temperatures greater than 130°F. Remember, any pressurized system can be hazardous to your health if it is not maintained and operated carefully and safely. 11-9

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CHAPTER 12 BASIC DIAGRAMS AND SYSTEMS In the preceding chapters, you learned about hydraulic and pneumatic fluids and components of fluid power systems. While having a knowledge of system components is essential, it is difficult to understand the interrelationship of these components by simply watching the system operate. The knowledge of system interrelation is required to effectively troubleshoot and maintain a fluid power system. Diagrams pro- vided in applicable technical publications or drawings are a valuable aid in understanding the operation of the system and in diagnosing the causes of malfunctions. This chapter explains the different types of diagrams used to illustrate fluid power circuits, including some of the symbols that depict fluid power components. Included in this chapter are descriptions and illustrations denoting the differences between open-center and closed-center fluid power systems. The last part of the chapter describes and illustrates some applications of basic fluid power systems. DIAGRAMS As mentioned earlier in this chapter, to troubleshoot fluid power systems intelligently, a mechanic or technician must be familiar with the system on which he or she is working. The mechanic must know the function of each component in the system and have a mental picture of its location in relation to other components. This can best be done by studying the diagrams of the system. A diagram may be defined as a graphic representation of an assembly or system that indicates the various parts and expresses the methods or principles of operations. The ability to read diagrams is a basic requirement for understanding the operation of fluid power systems. Understanding the diagrams of a system requires having a knowledge of the symbols used in the schematic diagrams. SYMBOLS The Navy uses two military standards that list mechanical symbols that must be used in preparing drawings that will contain symbolic representation. These standards are as follows: 1. Military Standard, Mechanical Symbols (Other than Aeronautical, Aerospacecraft, and Spacecraft Use), Part 1, MIL-STD-17B-1. 2. Military Standard, Mechanical Symbols for Aeronautical, Aerospacecraft, and Spacecraft Use, Part 2, MIL-STD-17B-2. Some of the symbols frequently used in fluid power systems have been selected from these two standards and are shown in Appendixes II and III. Appendix II contains symbols from MIL-STD-17B-1. Appendix III contains symbols from MIL-STD-17B-2. While the symbols shown in the appendixes are not all encompassing, they do provide a basis for an individual working with fluid power systems to build upon. Some rules applicable to graphical symbols for fluid diagrams are as follows: 1. Symbols show connections, flow paths, and the function of the component represented only. They do not indicate conditions occurring during transition from one flow path to another; nor do they indicate component construction or values, such as pressure or flow rate. 2. Symbols do not indicate the location of ports, direction of shifting of spools, or position of control elements on actual components. 3. Symbols may be rotated or reversed without altering their meaning except in cases of lines to reservoirs and vented manifolds. 4. Symbols may be drawn in any size. 5. Each symbol is drawn to show the normal or neutral condition of each component unless multiple circuit diagrams are furnished showing various phases of circuit operation. 12-1